I am a member of APDT I just want to give you a little insight into what you are looking for in a trainer. Being a trainer does not neccessarily mean they are also a behaviorist. Since dog training has no actual accreditation, joining the APDT does not guarentee they have to skills to deal with dog on dog aggression. To join the APDT, all you need to do is fill out a membership application and pay $100 in dues. I myself feel perfectly comfortable training basic obedience classes and dealing with basic problem behaviors such as chewing, jumping, pulling on a leash, etc., but would always send aggression cases to a more experienced trainer.

When you do your search, one thing you might want to look for is someone who has a CPDT behind their name. That stands for certified Pet Dog Trainer. It shows a higher level of knowledge and committment as the people who get certified have to complete several hours of seminar credits, training class time and pass a 3 hour test on dog training and behavior.

One thing I would highly recommend at this point. Tell your friend to never let the two together if he is not there to supervise. And I mean supervise - eyes completely on the dogs and not doing anything else. When he is supervising, keep both of them on a drag line (trailing a leash), that way if it looks like a fight is going to start - he can safely move the dogs away from each other.

Good luck on your trainer search. If you need any help finding someone qualified, please let me know. I might not deal with aggression myself, but I do know trainers that do.