Since he's outright telegraphed his plan to bring up Bill's cheating, it's not as if she doesn't have a half-dozen responses already prepared.
I suspect she'll just sidestep as gracefully as possible, rather than start a mud-flinging contest by bringing up Trump's history of being a scumbag. I suspect that there may be a "Mutually Assured Destruction" unspoken truce, where he might not go there unless she brings it up.
Also remember that this is a Town Hall, where they have to respond to questions from ordinary citizens. Somebody has to ask a question remotely related to the issue, otherwise he just looks like a lunatic bringing it up out of nowhere.