Also not a therapist, so take anything I say with a bucket of salt.
Apart from the great advice of having a routine (I was depressed for two years, had ribs showing too, before I managed to regularly get out the house and started improving), I guess you just have to change how you are looking at things. It'd be great to have good grades and if you manage to do it good on ya, but it's not worth your life. If you have to choose between dying because you're stuck in a rut and just taking a clean break, choose to live.
It sucks that your friend now has complications, and I hope they get better. I guess my advice there is just to be with them for what may/may not be a limited time left? Make memories while you can, and live on to carry that forward. I don't know what your living situation is like, nor do I have any great advice on the parental issue. I've had a single parent for 13 years now. But without knowing the details my best advice is to just talk to them about how stressed out they are making you, maybe they'll change/tone it down. At least you'll have got your say in.
Maybe also set aside some time to vent to someone about this regularly (whether that's a friend, therapist, family or internet strangers(though that might be a mixed bag)). Feel free to disregard everything I've said, no pressure. Just hope you keep on keeping on.