Aww, that would be sad overkill
I could never block my husband from my Facebook. Out of curiosity, I wonder what that would do to your relationship status on there?
Why don't you just block her? It will hide any posts she makes on his wall. You'll still see any responses that his other friends leave, but I can't imagine those being so raunchy?
Alternatively, would he be amenable to deleting her inappropriate posts? That's a lot different from asking him to stop talking to her altogether.