Im going to come from maybe a bot different angle...

I doubt she is trying to take your place with your son...she knows who his mother is and who it isn't.

Try to look at is as a positive...there is one more person who is looking out for the best interst of your son.

She might be a very nice person
She is not the OW...
She just happens to be with your X

don't give her more credit thna she deserves but don't take away the positives she may bring