Dead space 3 - friend's game crashes when trying to join lobby

Hello, I am having an issue with the co-op feature of Dead Space 3. I am able to host a game, no messages about strict NAT. I can see my friend trying to join my game, as his name appears in my lobby. However, a few seconds later, his game crashes. On his side, he is still searching for a match / joining my match. I am not sure what is the cause of this...

I am not able to join his match, I get a failed to join error. This is not surprising though as he has the strict NAT warning, he is unable to change this.