no one here can say what your friend was suffering from, none of us are professional doctors or anything

what I can say is their are anxiety disorders (such as generalised anxiety disorder), but I imagine, since you said 15 pills per day, their could be something else too- but honestly it's hard to say.

mental health is such a wide spectrum and their is so much that can affect the brain. the term mental health is very general. their are mood disorders, personality disorders, loads of stuff

as for why you wern't told perhaps your friend thought they could handle it by themselves, and they didn't want to tell you in case you didn't know how to take it

if you want to help the best thing you can do is just ask if their's anything that he wants to talk about, and support him through treatment. just tell him you're here to listen when ever he's ready to tell you