Every time, document date, time, what was said. Write it in front of the girl.
She could write this in a notebook, or even dictate it into a recorder on her phone. I would love to see the girl's face when your wife takes out her phone and documents what just happened. Who knows, maybe it's enough to get the girl to quit. As far as the principal goes, nothing works better than approaching and saying "Oct. 3, 8:23 am said..." "Oct 4, 9:14 said..." etc. Having a detailed description of everything organized and concise lets them know she isn't going to ignore it anymore. They will know she has evidence to give to an attorney or HR if they fail to take action.