Having a strong social support network is a good foundation for dealing with depression. My best advice to you is just be available for when he wants to talk. Let him know that if he ever needs to talk, be it day or night, you're there for him and that he can call you. Thoughts of suicidal ideation or self-harm can come at any time and he will need someone to talk to during those times.

Onto your other questions. The amount of time that he spends hanging out with any individual won't matter too much; having him be in company with people he enjoys several times a week is what is key. What this means is, it doesn't have to be you particularly but if he is hanging-out with others at least 2 or 3 times a week it would be beneficial.

The only boundaries that exist are the ones he implements. Generally, you'll just need to ask him how he's doing and not 'remind' him of his depression everyday, but how often you do that should be down to him to decide.

The best piece of advice I can give, is to observe him. If there is a drastic change in behavior or energy level, that is when you need to intervene and either try to get him to talk or just get him out of the house.

Overall, the above article is a good basis for coping with depression. The author hinted at, but never explicitly said, two key points that I feel the need to reiterate. If your friend is taking medication or is thinking about medication it can take awhile to take effect, on the magnitude of weeks or months, and that not all depression medication is the same - if one pill isn't working move onto others. Second, if your friend is seeing a therapist, again, not all therapist are the same. If he is not connecting with the therapist personally then his treatment is going to be much less effective.

I hope that helps.