I have a 4 yr old female boxer and part time 3 yr old male boxer(my moms dog i watch often), my friend has a 2 yr old female border collie mix and they have all played together with no issues ever. Then my friend adopted a 4 yr old female boxer mix and from the first time we introduced the dogs they didn't along. First meeting my female attacked the new dog, my dog is very social with dogs and humans, goes to doggie daycare, and has never gone after another dog. The second time we had the dogs together my female again tried to attack the boxer mix but I had her on the leash. It has been about 6 months since the last encounter so we tried again. I kept my female on the leash and I had the male boxer with me too. My male was playing with the boarder collie mix and then my friend walked in with her boxer mix on leash still and i had my female on the leash still and almost immediately my male went after my friends dog.
Is it possible for one dog to give off a scent that the others don't like? And is there any way to fix this. My friend and I used to run our dogs together twice a week and now we can't.
I doubt that the attacks are brought on because of a particular scent from the new boxer. Dogs react much more to body language and individual temperaments/personalities when it comes to getting along with other dogs. Introducing new dogs to established play groups can sometimes be tricky. Off leash, with little owner involvement, is usually best. Now that there have been several instances of fighting I'd stop trying to get them to all get along. The more often they have unpleasant encounters the more likely the fights will continue and can escalate in intensity.
Of course what you call an attack might not be what someone else would consider serious. It's really impossible to offer much insight without seeing the dogs interact in person. Very few dog fights happen for no reason. The humans may miss the signals the dogs are sending and the humans may inadvertently escalate the problem with tight leads and tense attitudes.
Not all dogs like every other dog. If you want to pursue trying to get them to all get along you should consult a behaviorist who can evaluate the dogs and the people and offer advice. Otherwise, leave the new boxer out of the mix.
Patricia McConnell Ph.D. has a booklet called Feeling Outnumbered? It might be helpful to you and your friend. She's a well known dog behaviorist and her blogs and website offer good advice and resources for dog owners.