S.D.
I think that your posing this question to yourself and your child is definitely the smart, sensitive thing to do. I think far too often in this world, we let our child do what they want without teaching them any lessons. Basically, we teach them "whatever feels right at the moment with no thought to what consequences may lay ahead as a result to our actions". I agree that he should invite who he wants to, but I would definitely make sure that it was for the reasons he stated, not because they had had a temporary fight. I would make sure that my son knew that this could damage any possiblity of future friendship (at least immediate future) as you did. If then he still wanted to leave them uninvited and I was even "weekly aquaintances" with their mother at best (meaning spoke to them on a weekly basis), I would make sure I mentioned the party to her. That way, there is no sneaking or hiding the fact and she can be ready to address hurt feelings on her child's side if they should arise.