Legal Question in Family Law in Texas

What does your decree say?

Unless there's a provision in the decree, you can't object to the people that your ex makes friends with.

I think you're talking about what's usually called a "morality clause", which is sometimes put into orders and essentially says that both parents agree that while the children are at a parent's residence, that parent will not have any overnight (or after 8 pm, 10 pm, whatever) "guests". If your attorney handled all the details of your divorce, it might be in there without you even realizing it, so look carefully through all the provisions and see whether it's there or not. If it's not, realistically, there's probably not much you can do about it unless for some reason this girl actually poses some kind of real danger to your kids (and being a generally "bad influence" you'd prefer they not be around because of her relationship with your ex is not going to qualify).