How can I help my friend who is in denial about having cancer?

First of all....sorry for your news. Unfortunately, besides supporting her and being there for her, there's not a lot you can do. You can try to encourage her to seek help. Get the doctor involved, too. If your SIL would at least agree to get the tests done then she can mull over the options. Without the tests, she really doesn't know what her options are. Support her as best you can. Facing mortality has to be the scariest thing in the world. Good Luck.