hi StillIntending

What might work best is for your friend to help the girlfriend get the best possible medication and therapy so that her depression can improve.
Perhaps continuing to make allowances and bearing with him till things improve with the girlfriend would be the best way forward.

All that can be done is to keep praying that his health will hold out to doing what he has to. You are right in saying that at the end of the day the battle with depression has to be fought by ourselves, but others can help.
There is Nami the organization for the mentally ill and they run peer to peer support groups which would give you extra support at this time and new friends. You can Google them and contact them to see if they run this service in your area. I hope and pray everything improves for you soon. Take care. God bless and best wishes from your friend Francis