She was not a very nice friend to me. There is a reason I stopped talking to her but I still can't help but feel uneasy. Anytime I hear of someone who I knew at one point or another, it bothers me when I hear about them dying or having already died.I'm sorry to hear about your friend, cancer is a total bitch.
You don't have to feel anything, you can sympathize and understand that it sucks, but you're not obligated to feel bad or good. It's an awful disease and it sucks to suffer through it, but you don't suddenly have to make yourself her best friend just because of awful news. It's unfortunate, wish her all the best, and carry on.