When supporting someone struggling with mental health difficulties it is difficult to know what to do. Really it is as simple as this:
Let them know you have noticed a change in them
Let them know you are there if and when they want to talk
Sit with them even if you don't speak
If they want to speak just listen
Just be there
That is all it takes.
Everyone wants to find a way to 'fix' someone, with mental health there is no quick fix all you can do is let them know you are there for them whenever and for whatever they need.