Seconded. There are so many things that you can do to help...from helping to research / make appointments with therapists (it can be very tiring to try to find one with availability, and folks w/ depression don't have energy)...to helping them see their primary care provider...to being understanding and supportive.

That said, know that if, after many months / years of you attempting to help, if the partner does not get involved in any way in trying to get better, there can be an issue of enablement on your end. If you ever get to that point, read the archives for Captain Awkward, she has a lot to say on that subject.