I need help with a very specific, and unusual NAT issue.

I'm aware there is a rule that you shouldn't post a question that can be Googled. But, Google doesn't seem to have the answer for this one.

I have been having NAT issues with a certain player. When this player joins a party I'm in I always lose connection to the party and I also lose connection to whatever game I'm playing at that time. When I join back I can't hear anyone in the party due to NAT errors and within a couple of seconds I will lose connection again.

This is extra weird because of the fact that I've been friends with this player, and I've spoken to him on a regular basis, for around three years. We also both have a Type 2 NAT and stable internet. I haven't messed with my internet and neither has he. This just came out of nowhere.

I've tried various methods of troubleshooting for this issue. I have restarted my router, my console, I've set a static IP, I've forwarded ports, and I even put my PS4's IP in to my router's DMZ for a short period of time to see if something would change. When I contacted Sony support about this all they had to suggest were methods I had already done. The only thing that I didn't do was swap my NAT to Type 1 because as of now that's a possibility for me.

I really need help with this issue because this person is an integral part of my friend group. And not only can I not talk to him: But, I can't talk to my other friends while he's around either. Does anyone have some sort of idea or fix that I could try? It's very important to me.