Just had this discussion with my MIL, Betsy.
I certainly know the old Miss Manner's piece of advice:
"If you don't have anything nice to say, don't say anything."
While this certainly is a good adage to follow if you are upset at someone, this is wholly inapropriate to someone with cancer. Why?
Because saying nothing can be taken as a rejection and/or distancing of oneself from the person with cancer.
Just tell them you don't know what to say.