A very close friend of mine, let's call him David, in his mid-thirties has just (1 year and a half) recovered from a very hard and horrifying for his environment state of drug abuse and isolation that lasted more than 10 years in a culminating manner.
With a strong physique (former martial arts athlete) and a preference for vigilance-inducing drugs he was not easy to cope with. Things got very tense as the drugs started causing him hallucinations with a more lasting effect and with violent outbursts, which had legal backfires to him.
Now after a close relative of his took him with her, David has been reducing the drugs he uses, to the point that I think that now he is not using them at all (more because he realized that they started making him really freak out and less in respect of others). His abstaining was accompanied with an effort to learn to play music (he played the guitar as a teen), which now takes almost all of his day. He is trying to learn different instruments at the same time and he really tries hard, attending classes for mandolin and percussion and spending many hours practicing.
The thing is that my wife has been playing mandolin since she was a kid and when we all meet, he tries to
check his progress with her get a compliment for his progress from her. But what my wife thinks is that his mandolin teacher, a woman younger than him, is approaching him in an understandable at some point but insincere way, not telling him the truth about his progress, not giving him basic guidance when he clearly lacks it, guiding him to practice stuff he is the least capable to practice, suggesting that he would be helped if he bought a better instrument, when she doesn't even tell him to clean his instrument, almost umplayble from dirt on the strings.
It is clear that David has shown mistrust for her competency, trying to use tricks he sees in youtube that can by no means truly handle, or he has made her feel awkward, requesting that she plays stuff she hasn't practiced etc and in general he seems impatient and trying to achieve more than reasonably possible (like trying to make up at once for all the time gone) and even the thing with the new mandolin purchase could easily be something he suggested. That is why I called her behaviour understandable at some point, she clearly doesn't want to confront him with the truth or to put pressure on him and herself or she feels frightened by him, so she gives completely random lessons, with exercises in books highly advanced, and seemingly she just lets him do whatever he wants as long as she keeps her lesson (its a private music school).
So while David has made some progress (he exercises many many hours and that would happen either way) it is clear that his mandolin lesson is a complete waste of time. My wife vaguely made some hints for the whole thing but his frustation caused her to stop and the next time she avoided telling him what she thought (kind of like his teacher).
The thing is that in his other lesson he seems to be getting it better. The percussion teacher is a man and it seems that he is getting David to start from basic stuff,
which is hard for David but he is handling it which is hard for David because he has to try very hard and not in the way he was used to (to do the things as he wishes, like playing along youtube songs without listening the least to what he is playing or playing a song at whatever tempo as if music is all about speed and nothing else). The percussion teacher has imposed some methodology on him and David is getting very slow results (but he is getting them) instead of a progress that is mostly in his head. I guess as a man his percussion teacher is not that much worried to confront him.
So I would like to say or give a hint to David about his mandolin progress and his lesson, because I believe he is kinda getting deceived. How could I do that in a way that will help him go on (perhaps with a better teacher) and not hurt his feelings or utterly frustrate him?
EDIT Thank you for your answers so far. I rephrased some things to make some points mentioned more clear.