It was mentioned specifically in the trailer that it's a prescribed therapeutic tool. I can't be arsed to watch it again and tell you the exact moment, but it's there if you look. I distinctly remember that bit.
It's only "mentioned" when we see Jodi Foster (the ex-wife) read the card handed to her by Mel which, for all we know, Mel made-up himself and it wasn't done by a doctor.
The narrative in the trailer suggests the puppet's personality and the resulting therapy comes out on its own and not through a doctor's suggestion.
Reading some reviews it seems as if this case, he goes through this phase not under the direction of a doctor but of his own volition. Again, it's my belief that you are doing this your mental health is not getting better.
I am willing to "forgive and forget" Gibson's tirades if he manages to entertain me again -I can still watch his older movies and enjoy them- but this movie doesn't look like it's going to be one I'll be dying to see.