NCB, I don't think there is a thing you can do about it. I had the same issues with my X and his gf (who started off as the ow). They now live together, and there are even times when he has visitation, the kids are with her and not with him. They are not married but live together and share the same bed.
It sucks. The law is not about morality, but legality. So, unless the bf does something that could be considered illegal / puts your kids in danger, there are no consequences. You can't control who she is with or what she does with the boys while they are in her care. All you can do is do is focus on being the parent you think they deserve while they are with you. I'm sorry that there isn't anything more...
A letter from you will do nothing except make matters worse. WAS's usually don't even consider consequences from outside sources like therapists, since they have already justified their actions to such an extent. Anything you try to direct through her will only fall on deaf/hostile ears.
