[RobertoG]

" I feel there is really no evidence other than the testimony of one person"

There is not even the testimony of that person. They don't told you what she say, they don't allow you to confront her.

For all that you know they could be making up all the thing. I don't think this is the case, but it's a possibility when everything is done in the dark.

Don't take this as advice, please, because I can't predict the results, but in your position I think I would feel compelled to confront them. Maybe with the help of some worker representative if that it's available to you. What I would avoid is to confront the woman, that could backfire very fast.