That's a tough situation! We have neighbours who have two children. The younger one gets along well with my daughter. The older one is, well frankly a bit of
a nasty brat. I've had to tell her to go home when she's being obnoxious, and then had her turn around with her hands on her hips and say, "You don't get to tell me what to do!" In my own backyard no less!
In the past my daughter has tried inviting only younger daughter to her parties (with only HER name on the written invite), only to have the parents drop off both daughters. Clearly they come as a set.
It's been several years since my daughter has tried inviting them to a party. She sticks to her school friends now.
Maybe since you know that this boy likes your daughter, but dislikes your son, you need to make it clear to his mother that your children aren't a matched set. They don't have to do everything together, and you won't be offended if she gets invited somewhere he isn't. (Unless you would be offended, that is!)