This is interesting. I had one girlfriend who would NOT admit she had depression. She would withdraw for days or weeks. Sometimes I would buy tickets to expensive events and I'd show up to her house to pick her up and she would be laying in bed, refusing to go. Rather than letting me know hours in advance she was "sick" (it was depression), she would wait until I arrived to tell me she couldn't make it. Sometimes we were supposed to go to dinner reservations and she wouldn't even answer the door even though I could hear her moving around in her place and she'd claim she was too sick to answer.
You can be understanding but still maintain boundaries and respect with people. I have kept a really strong distance with her since the last time she did that and now our friendship is decent. Because we are not as close she will only reach out when she knows she feels happy enough to see me. And, that works so much better for us.