Hi Lmo!
I see four components to question #1 (so far, I should say

):
1. WAS/IS depression his primary issue? After two years of being on the same board with each other, it seems like that would have been pretty much a constant theme if it were accurate. If now, two years later, she is zeroing in on this as the reason and almost having to justify/describe how it could fit, it doesn't seem all that... fitting. I do think it would be hard not to have been pretty sure of that long ago, at least if they spent a lot of time together.
2. If it's not accurate, is it best not to point that out? I think we have to be open to picking up on potential clues about ourselves and any things that really may be hard or impossible for others to deal with. Because otherwise we can't grow, and have a better experience next time. Now, of course we don't know if this guy was a prize, a good or fair person. But in general, she should at least be open to seeing patterns in herself, that may indeed be there, things that really may need to change. So letting herself pin blame elsewhere isn't helping her ensure her own future happiness with someone else.
3. Was she strong enough to hear it? I don't have enough experience to know how that kind of thing breaks down regarding *disorders,* and if/when it is best not to risk upsetting someone. But, while she does sound like a sensitive person, she signed on as the S.O. of one with a mental/emotional problem, not the afflicted one. So ostensibly, I'd imagine she would be (and that you'd assume she was) reasonably up for the truth or should be, even it was potentially unsettling.
4. Are people with depression often moved to break up with someone based on that? That's trickier, so I'll talk about meeee.

I do think it happens, at least I can feel it in myself. I don't have depression as a primary issue, more like an occasional ramification of primary ones. But from the outside looking in, I think people could easily label my whole "complex" as depression, for the way it holds me back and because one thing or another always feels out of sorts, and I can't plan, and need to keep to myself about it and be alone a lot. It is very hard for me to let someone in and let them stay in.
If they met me in what seemed like good times I know it won't last and feel like I'm defrauding them. In long term relationships I've gotten to where I can let someone in at most times, and I am not necessarily "down" in my bad times. But I usually don't want to go anywhere, and can stay in that mode for a long time. So I've broken up with people if in their heart of hearts I knew they craved an active, spontaneous life; and progression and more time together; and I just couldn't stand how it all made me feel after awhile. But! I am different from many people in that while I do like people and am good with them, I am also perfectly content alone, indefinitely. Completely alone. And I don't feel a need to talk. So with me the craving for privacy in my really bad times can easily outweigh the benefits of the relationship, and I may very well break up with someone... feeling it's definitely for their own good too.
Question #2, you! Well, obviously I'm pretty new, and I don't read a large percentage of the board at all. So in reality I haven't seen big concentrations of your posts. I do know I have seen very good sense in many that I have read, and that my gut impression from that has been that you're sharp, friendly and open-minded. But to "educatedly" answer what you've asked, I guess I'd need to have seen more or to have been here longer.
Tell ya what, if I get up to speed on a more diverse selection, I'll check in again then! Regardless, it does seem that your heart is in the right place, and that if you "slip," you're open to hearing it, and you care about it. Those are the most important ingredients I'd imagine...