I can only speak from personal experience, but I made enormous improvements on my own with a book in a pretty short period of time. I'm now at the point where I'm looking into joining a tutor-led group, because I think I've made most of the easy gains from pursuing it on my own.

If you're the sort of person who can, for example, hunker down and learn a programming language over a few hours under your own steam, I imagine you can probably do the same with drawing. For me, the experiences are quite similar.

Hi Adrien ! Welcome :)

Feel free to introduce yourself on the welcome thread

Short answer: I don't know.

Longer answer: I am actually very interested in seeing how you progress without menthorship (for my own empirical study - I've seen how people respond to an intensive 8 hour seminar, I haven't seen how people improve with 8 hours of their own efforts. Control group would be useful)

I will note that I do not have any education in teaching - I ran this workshop specifically to gain experience in it. If your friend has college level (or equivalent) art training, then e will probably be about as qualified as I am to give you feedback. The question is the value of "occasional feedback" vs "intensive dedicated training."

In the workshop, I gave constant feedback and occasionally demonstrated my own drawing technique. Participants felt that doing so was very helpful, and I believe it was, but I don't know for sure how important it was. (We did notice an effect where shortly after watching me draw, people improved at drawing for a little while, although the effect didn't last indefinitely, presumably as the memory faded and their old habits returned. In the next session I intend to demonstrate once every half hour to reinforce the benefit, and then see if the improvements seem to stick longer.)