I would actually be worried if that happened to me. Personally I have never had a dog show aggression to me or my husband, even our rescues. But I think that we are fairly strong around our dogs.
I however would not recommend any "quick-fix" to some one I do not know or their dog. If it was my friend, I would suggest she visits a dog behaviourist pronto. to access the problem and to see what is happening.
I can only compare to what I would do in such a situation. One of our newf rescues was possibly people aggressive, so we asked help from someone who trains dog/ behaviourist. He checked her out and she was not aggressive at all, just scared. But she never showed any aggression to us, even when we had to physically pull her out of a dog fight.
So if I was your friend I would not tackle this by myself, but I would do something and quickly. And I would most likely get him neutered. Even then it would take a while for the hormones to go and sometimes the habits need to be redirected.