Well, I think it depends on the reason- there are valid reasons to distance yourself from someone who is "in need". For instance, could it be that they can't cope with all of this?
Maybe they are having to put themselves first at this time for their own physical or mental health? Maybe they don't feel they can actually help this person anymore, and are only likely to make matters worse.
It could be that they just can't be bothered and have "abandoned ship" of course. Which isn't nice, but your friend may well be better off without them if they simply can't be bothered.
You asked for stories. Here's mine.
I had a friend while at university, who began self-harming and displayed destructive behaviour (sleeping around, drinking too much resulting in her doing stupid things- and a few very mean and horrible things on a couple of occasions). I have heard through the grapevine that she has since been diagnosed with borderline personality disorder.
Her self-harming worsened around the same time as my dad became seriously ill (he was hospitalised for almost a year, spending nearly 4 months in and out of ICU) and as I was at university, we still had course work and exams etc.
I simply didn't have the emotional or physical energy to deal with her problems, my own problems and studying. I had already begun to tire of being one of her "shoulder's to cry on" (not to mention the emotional manipulation), keeping an eye on her and trying to perform damage to limitation after she had done something stupid/embarrassing/mean- and I had been considering distancing myself, as it all felt one-sided most of the time.
I guess my dad becoming ill just hastened what would have happened anyway, but I did quite suddenly just stop being there for her after one particular event where she self-harmed (she had done more damage than she had intended- it wasn't a suicide attempt) and I found her- this was shortly after I found out my dad was back in ICU and given less than a 5% chance of survival. She knew this. I just couldn't take any more and I had to keep away for my own sake.
When I backed off, a couple of other people did too. After talking it through with some friends, it seems a number of them were feeling as I had been, but were worried others would think they were horrible people if they stopped being friends with this girl, so carried on- but had noticed that there had not been a nasty backlash against me, so felt more able to walk away.
A few people did judge me harshly- used similar arguments to you. I'm no longer friends with them, but I don't think judging people is going to help you or your friend.
I understand you are trying to stick up for your friend, and it sounds like they are lucky to have someone who will be there for them. You know these other people and maybe they don't have a good reason and are simply being flaky when things get tough. Though they have stuck around for quite a long time for "flaky" people (I'm assuming they've been supportive for the 2 years of worsening mental health issues).
Has it occurred to you that one or more of these people do have a good reason that you aren't aware of? Maybe they don't have one "good" single reason, but just feel overwhelmed and cant cope anymore. It doesn't necessarily make them mean, selfish or heartless.