They need to forgive Barb, because Barb's demands are medically necessary and she's coping with what sounds like a very recent diagnosis. This is the sort of thing that brings out the worst in people. (I'm assuming by the fact that Barb is a friend at all that this behavior is not normal for her.)

However, they're very likely not going to be able to ensure a house free of cross-contamination, and they definitely need to say that to her, in a nice, judgment-free way.