Nothing rips apart a relationship like an icky medical diagnosis. First you have to deal with your so called 'friends' who head for the hills at the first sign of anything yucky, especially the word 'cancer'. They are not your friends. Forget about them.
Then you have friends and family who obsess about your ailment and call and email constantly for the latest worst of wisdom imparted by your medical team. They often offer their 'advice' in return saying things like 'I can't understand why your doctor hasn't given you [insert the name of the treatment given to their neighbor's dog walker's cousin's hair dresser when they had a completely different diagnosis fifteen years ago]. They also want to know every time you have the least minor issue so they can be 'informed' about how you are doing. You have no idea why they need to know so much and what they are doing with all this information but feel invaded by their constant barrage of inquiries. Again, they are not really your friend either.
The people who are your friends are the ones who stop by and visit, call and just listen, and treat you like a human being, while you are trying not to lose your lunch after your latest infusion. Hang out with these people.
Finally, you have the person closest to you: your spouse/partner/best friend. Too many people find themselves suddenly single after a cancer diagnosis, in particular. I know we had our ups and downs with my breast cancer diagnosis. But I can't tell you how many friends have spoken of their spouse's lack of support, or even departure.
My husband has been wonderful. For the last three years, he has given me my weekly injections for Rheumatoid Arthritis, because I cannot deal with the idea of giving myself a shot. He accompanied me to most of my doctor appointments after my breast cancer diagnosis, where he was allowed to see everything other than my weight and any pelvic exams. He came with me to every chemo infusion where we played scrabble during the infusions (and he would delight in winning when the drugs kicked in and I could only form two letter words). He only stopped going with me when I insisted he couldn't keep missing work for weekly infusions. Now he will go with me to any medical appointment I request him to.
Unfortunately I have friends who are forced to go to difficult appointments alone or are forced to find rides for infusions or other times when they are not in shape to drive. Their partners find excuses as to why they can't help out. Or have left them completely to cope alone with bad medical news and the accompanying job loss and reduced income.
All I can say if you have a friend or partner coping with a medical disaster, don't walk away from them. If you have problems emotionally coping with their medical problems, please do not leave but try to face your concerns and become a better you. Its you who has the problem not them.