Well even if they weren't best buds, the Federation and Klingons had been at peace for awhile by that point (they even had an officer exchange program, for god's sake), so I can see them coming to the other's aid when needed.
Plus I imagine the Klingons didn't need much convincing anyway to fly into the Neutral Zone and join in a possible war against the Romulans. They live
for that stuff.
I have to say though, as good as the Defector is, it now seems painfully obvious from the outset
that he was just being set up. Once you see how the Warbird was hanging back from the scout ship, you have a hard time buying pretty much anything
the guy says.