Edamame, all my best wishes for your mom, it's very terrible to hear that it is such a serious illness, I really pray that it will be as slow as possible.
I know it is not my place to say, but together with "normal" therapies please look into alternative therapies, like nutrition, acupuncture etc, I was diagnosed with an incurable problem doomed to spend my life in a bed and instead I have almost a life, although I am sure I got lucky. Anyway, many don't think that small things like diet changes or magnetic fields or chemical pollution (just to mention a few) can make a big change but some times they do. Don't hate me for bothering you please!
About your post, conventional friendships that don't come from a deep connection of the heart tend to dissolve as soon as one is sick, most people just can't deal with that. It's a fact of life, very sad but there isn't much to do about it, except maybe giving a good example of acceptance to the next generations.
In our society everything that is sickness and sadness must disappear,and we are trained since birth to feel uneasy in front of these events. I don't think it's much consolation, but it's not personal or out of cold heart, it's because of their training.
I had that twice, once some years ago with the above mentioned illness, after less than 6 months I didn't have one soul to speak with, and this year when I made the wrong choices and accepted "conventional" friendships, the "how is it, wanna go to the cinema tonight, see you around" kind, not from the heart, and I was at home alone with pneumonia for two months without even as much as a phone call, and one ***** (forgive my french) after she'd asked me to go out and I answered " can't I have pneumonia" managed to reply: oh, that is such a drag! and then disappeared.