My 6 year old son's almost stepmother is giving him baths and washing his privates. What can I do?

Ok... A little background my son is 6 years old almost 7. He is very capable of bathing himself, dressing himself, he even likes to cook for himself (Before I get reamed with comments about him cooking for himself it is the microwave obviously and only stove with supervision). He is a very independent child. Lately though he has been acting like a baby... and wanting to play baby. When I asked him why he liked playing baby and who plays it with him he mentioned his almost stepmother plays this with him. When I asked what they do when they play baby he said she dresses him and washes him... So I asked again like washes what? His response startled me he said she gives him baths and washes his and then smiled... I asked his father about this and he confirmed that she does give him baths and that she does wash his privates. Okay so a little background my son lives between his fathers house and my house (we have joint custody across the bored). His almost stepmother does not live with him but obviously they do spend a large amount of time at each others houses. I asked his father to not allow anyone other then us (his biological parents) to give him baths or allow him to be touched by anyone other than us when he is naked. His father then informed me that he sees nothing wrong with it and he will continue to allow her to help with baths. I do not feel comfortable with her doing this. In all fairness to her she seems nice she is 23 I do not really know her too well, but one huge issue that has always raised red flags for me is that she will babysit kids for free, and she would even bring these kids she would babysit over to her boyfriends house (my sons father) when they were going to be alone for the weekend. She even owned her own car seat and window shade. After the first week of meeting my son she asked him to call her mommy. Now before I get the you are just jealous, I have done a lot of self examination and in the beginning it was extremely hard but I have come to terms with it. So I guess what I am asking is my mothers intuition tells me there is something off about this woman. And I really do NOT want her bathing my son he is almost 7!!! Is there anything I can do to keep this from happening as his father does not agree with me and said he will still allow it. Also I am I over reacting? I figured I will ask this since I am sure you all will tell me anyways if I am :) Thank you for your answers concerning this subject.