Your dogs are in no way at fault for the behavior of her dog. If her dog does indeed have a problem with boxers, she should never have mentioned trying to play.
I do know a person whose chocolate lab was attacked by a golden retriever when he was a puppy and he has ever since hated goldens, or any dog of that approximate size and coloring. He's 13 now and has taught his younger 'brother' who is 9 the same trait! So it is definitely possible for dogs to hold fear/aggression towards specific breeds or colors. (To be fair to the lab, I do not think his owner had any knowledge about socializing at the time, he was her first dog ever, so he never had a chance to get over his fear.)
- Corinne -
Gus: 8/6/2010, boxer/lab mix
Toby: 3/1994 - 4/26/2010, at the bridge