I'm in shock that anyone would be that socially inept.
I'd be really tempted to say something snarky, too. Or maybe just, "Sorry, I really can't invite anyone else to my party. But I'm concerned that you've created a situation where UF's feelings will be hurt. You see, we generally don't tell people about parties to which they are not invited, just because peoples' feelings do get hurt that way. I'm sure it would be helpful if you held a party and invited them - perhaps you might want to do so on the same night as my party to ensure they don't feel bad."
*snarl*
-Ivy
Parenting two active, wonderful boys
This is your world. Shape it or someone else will. -Gary Lew