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I think you were too harsh. Improvisation is itself a useful skill, especially so if your friend has an interest in jazz. Trying new things can also help with composition - I'd imagine most music doesn't spring from the composer's head fully formed. You may find an interesting melody or rhythm when just noodling around that you want to keep for later. And above all, if your friend spent 4 hours playing the piano for his own enjoyment, that's a perfectly legitimate reason in itself. Intensive practicing can be draining, there's almost certainly some mental benefits in taking a break and just having fun!

What your friend is doing is absolutely essential. Your friend is developing deeper and deeper layers of understanding music. He's going to places and seeing things you don't see. I suggest you start playing by ear and improvising and exploring the world of music too. It's not a guided tour, it's an exploration.

Babbling is a stage in language acquisition. (Wikipedia)

You need to babble with notes, rhythms, scales, chords, chord progressions, tones, dynamics, melodies, motifs, modulations, fingering patterns, timbres, instruments, ensembles... Higher levels of abstraction are built on lower levels, and you need to discover them and experience first hand how they work.

Some amount of systematic goal-setting is good in the long term, but you said your friend did some "actual" practicing. A few hours is nothing.

"Doodling" sound a bit dismissive. "Improvising" sounds more serious.

It would help to know more about just what this playing involves.

But, if we assume that this playing is musically interesting. I think it is valuable. But that value will depend on what your friends goals are and they go about practicing.

Surely everyone will want to have good technique, but time spent improvising is about learning how to develop new ideas spontaneously. You will concentrate on other things beside technique. Things like recombining and varying musical patterns.

It seem important to note many good players cannot improvise, because they didn't devote time to developing that unique set of skills. Isn't that a shortcoming for a musician too?

Jazz will be the obvious case for piano improvization, but there is a tradition of improvisation in classical style too. Take a look at these for starters...

• Techniques of keyboard improvisation in the German Baroque and their implications for today's pedagogy


• 
  Partimenti of Durante - especially look at the suggested "styles" to improvise the right hand part over the notated bass.

If you're practicing, then doodling or improvising is perfectly fine! It actually can be a form of practice because it helps with coming up with new ideas and freeing your hands more on the piano.

Also, depending on the type of improvisation, it can be almost just as valuable as practicing for real. If you're able to incorporate scales and triads and other technique into your "doodles", then that can have some benefits for technique. Also, improvising on a song you already know or one you're still learning can have benefits too as you're getting more comfortable with the song. Also, improvising with a new genre of music like jazz if you're classical or doing ragtime can help you with learning different types of music.

The only "doodles" that aren't valuable are playing Hot-Cross Buns or something like that that's way too easy. Otherwise, you're doing good! You can't be strictly serious all the time on the piano, or you'll learn to hate it. Have some fun!

Your friend is definitely not wasting his time. Really learning how to improvise (which is a building block toward being able to compose music) takes an enormous amount of time. Most of the better composers I know spent a lot of time "doodling" on their instruments before moving on to a more structured process of composition. If your friend is interested in jazz or in composition, then he is on the right track.

Once there was a guitar player “doodling” in a music studio when an engineer walked by, heard the doodle, and was immediately impressed – so much so that he had the guitarist record it.

The guitarist? Eddie Van Halen. The doodle? The legendary neoclassical shred “Eruption”.

As someone who designs artificial intelligence (AI) for a living and has dabbled in composing music, I think I can offer a different perspective.

Think about an auto piano. It can flawlessly play complex music, because it was trained or programmed to. But it does this without any understanding of music theory. Likewise, a human can memorize scales and learn to play music simply by building muscle memory, but memorization is not the same as understanding.

The goal when designing an AI is to give it the ability to acquire a deep understanding of a topic on its own, rather than relying on rote memorization. A common way to do this is to have the AI generate its own music, and by doing so, it learns something about the underlying aspect of music composition.

Similarly, when people noodle around on an instrument, they learn "what sounds natural". They are essentially learning music theory without any formalization to their understanding. Noodling might teach someone about chord progressions when they don't even know the names of the chords - they just learn which patterns sound good after other patterns. It is a perfectly acceptable way to learn.

IMO there should be a balanced program for each practice session, in which you start by posing some restriction to yourself, and gradually loosen it.
For example, you first practice scales for several keys, and several figures transposed in several keys.
Then you play iReal Pro and play along, utilizing the figures you just practiced, whether consciously or not.
When you have "turned on", play some of the standard pieces from the Real Book and render solo passages as you see fit, without help of tracks.
At this point, you are likely to be good spirited and full of inspiration, and play freely and record.

Listening to what you played also helps.
You may think your improvisation to be "inferior" than great Jazz pianists, but you may not realize the deep reason.
Try to find that out, for whatever styles you are endorsing.
In the beginning, most likely you can't, since you have not developed taste sharp enough.

My advice is that, when learning everything, you must first be constrained in some way, and that, somehow paradoxically, helps your imagination.
You avoid the problem that you are "stuck" in the same level and repeating wrong or mediocre patterns, by adding some "external force" to drive you out.
When you force yourself use some figures, you learn new ways to exploit them.
When you are determined to play within some harmony progressions, you may incidentally find some progressions that you don't like before actually sound good, and so on.
And as you remove the restriction, you get more creative than before.
This too can be said of four-part harmony, counterpoint exercises, and probably other field of expertise as well, I guess, like mathematics, but I have been digressing.