I think it's good that the OP gave her friend a heads up that she was going to be uncommunicative for awhile. My BFF is bi-polar, and when she goes into a dark place and doesn't reach out at all (even just a quick text or email), it's frustrating and scary. I may be a "normie", but I totally get depression - my mother is also bi-polar. It requires a lot of patience though, to be close to someone who suffers from this. I lost a close friendship years ago due to depression. When she shut down completely, she cut me out - and when she got better, she never acknowledged how she treated me (and others). I am a very understanding and forgiving person, but I don't absolve people with mental illnesses from any responsibility when they hurt others.
I 100% understand Rora's hurt over this, and I think it's impossible to know what her true intentions were when she contacted the boyfriend. If my best friend went to a dark place again and cut me out but still communicated with her boyfriend, I'd be contacting him to make sure she's ok. If that makes me rude, that's cool. She's attempted suicide twice, and this (for me) would be a safety before etiquette situation.
Rora actually ending the friendship seems pretty over the top though, so I think it might be safe to say that she doesn't have the patience and understanding to be good friends with someone who's depressed.
OP - if you do have an interest in saving the friendship, I think it's fine to contact her, apologize for not being there and explain again what's going on with you, and if she responds well then you can chalk her past behavior as her having a bad day. If she responds poorly, I'd give up.