My friend's daughter is the devil....?

She is mean and nasty. Her favorite saying.."You're not my friend anymore!!", "I hate you!" and her newest..."Dum-Dum". She has said them all to my son. I kept them apart for a few weeks last time she told my son she hated him. She was talked to and punished by her Mom. So, we started seeing my friend and her daughter again. The devil actually turned into (somewhat of an) angel for a while. Then the other day, she started with the favorite sayings and the new dum-dum one towards my son. So, we left. I told my son we weren't going to hang out with her for a while (again). Now, this devil child is having a birthday and we have been invited. We have yet to buy a gift (which I really don't want to now). My son says he wants to attend the party, but I really don't. I know this is my son's decision, so we are going. Problem is, this imp doesn't deserve a gift, and I'm having a hard time wanting to spend my own money on this brat. Any suggestions?