Ugh, I've had my share of biters and I Loathe it!!! I've never given up on any of them but I did lose a dcf because of 1 biter. And I told the dcf I probably would do the same thing as they were doing(finding other care) but I knew the biter's family was a wonderful family and I didn't want to lose them either.
As for your dcm, it just sounds like she was expressing concern about upcoming pictures and not towards her child getting bit? Maybe I'm wrong but that's the impression I got from what you said. If she was mad because her child got bit then I would definitely bring up the fact her precious one is also a biter and she, better than many, should understand about it.