Giant Bomb! I have always come to you hooligans for help, and i'm wondering if I could be assisted in this problem i am having.  
 
this friend of mine just got Saint's Row 2 and we want to blow the shit out of everything in the very well made Co Op mode. now, I have beaten the story mode with my friend on Co Op, so i have no real problems with Co Op before.  
 
i've only played co op once with this guy, and yesterday when we tried to play, i got a strange, un-detailed error.  
 
"You Cannot Join"
 
 really? this is Street Fighter IV all over again.  He had the same problem joining my session, too. 
 
it's not a problem on my end, since I played co op with another friend shortly after, and it's not a problem (i believe) on his end, because he has no problems with his internet as far as i know, and plus we play Red Faction occasionally so we don't have much problems connecting.   
 
Thing is, i don't know much when it comes to internet problems and stuff.  has anyone had this problem before? if so, how can I fix this?   
 
I'm playing the PS3 version. 

Try out the Test Xbox Live Connection in the main options, it could be a NAT issue. Get your friend to test it on their end.