Worried my kid and my sisters kid won't talk again

My sister and I haven't seen each other in years. We both have sons who are 17. We got to see each other yesterday for my brothers wedding and it was a disaster. My son has a girlfriend who he's been with for a year. My nephew brought his girlfriend to the wedding. When my son got to meet his cousins girlfriend.... She was my son's girlfriend. In case you're confused she was cheating on my son with his cousin and had no idea they were cousins. My nephew did not know. He had been long distance with her for 5 months. That was extremely awkward and how that ended was my son telling her "I love your makeup for the wedding. Let me help you with it." And dumped his ice coffee in her head. Not a right thing to do. But neither is cheater so whatever. He left to his dads. He didn't even stay for the wedding and he's really heartbroken. And doesn't want to talk to his cousin but his cousin didn't even know. So I'm not sure why he's mad at him. My ex is saying to give it longer than a weekend. Just give him time to calm down and he will realize his cousin didn't do anything wrong. Do I keep reminding him his cousin is not at fault or let him have the space to calm down.

@UnicornsAreReal I don't think his cousin knew because he was questioning her too. Also he did not invite her because she had already told him a while ago she was going out of town so he saw no point in inviting her. I don't think she knew they were cousins because me and my sister have not seen each other in a long time. She kind of seperated herself from the family for a while. So they never really had much contact