Cancer (yours or anyone else's) doesn't change anyone's personality, but it sure puts stress on the fault lines.
I've been in this space for much longer than I ever expected to be around, and I can genuinely say that once the shit really hits the fan there can be some folk who are mightily disappointed in their partners, friends or family, but very few who are honestly surprised.
It's a brutal process, and relationship issues are the last thing we want to deal with when it feels like the rest of your life is going down the gurgler. Best advice is treat the situation like you would any other life threatening catastrophe--preserve life first. Don't waste too much energy on consoling the bystanders, if that's what they choose to be. They will wander off, decide to help or (the true fuckers) record the lot on their smart phone and expect to be treated like they are the traumatised party. Hang in there and keep your eye on the main game--survival. The rest seems to work itself out. MXX