I have a friend who is dealing with breast cancer and the many painful repercussions of reconstruction surgery. She has said that while she appreciates the effort to raise awareness and to raise money to fight cancer, the last thing she wants is to be constantly reminded of her cancer by being surrounded by lovely pink ribbons everywhere. This is just one individual, and your SIL's mom may have an entirely different response, but you didn't say exactly that it's the cancer victim who is requesting this quilt. Maybe it's meant as a surprise for her. There's no way I would make that quilt for her without knowing how she feels about the theme. It could be donated for a raffle if she doesn't want it, but just in case there's any chance she might feel as my friend does, I thought I'd put in my 2 bits on the subject. I'm sure that whatever you decide to do, the thought will be appreciated.
Perhaps other cancer survivors would like to weigh in on this topic and we could get a more balanced view than just my one friend's side of it.