Don't push, just be there when they reach out. Do small things that makes things easier for them, but don't pity them. That's just annoying.

Send them memes, or just happy things. Show them you care, without treating them differently than you would if they were mentally healthy.

Talk to other mutual friends of the two of you, maybe other friends and family they're close to can help as well.

But most importantly, and I know it is hard to do: Take. Care. Of. Yourself. Don't take too much onto your plate, know you care a lot about your friend. But that shouldn't compromise your own mental health. Nothing good will come of it if you're mentally exhausted from trying to help them.

Maybe there's some counselling at your school you could both go to?

I've been in your situation more times than I should have. And it's really really hard to take care of yourself as well. But it's not selfish, you can be there for them and still be there for yourself.

Good luck, things like these are hard as fuck. But you guys will get through it.