I have 5 dogs and 1 foster dog and none of mine can go to a dog park only because 5 of them are highly reactive to other dogs. All my dogs are sterilzed but there are dominent issues with each (except for one who is an absolute angel except he is a target for other dogs based on his submissive nature).
My husband is a certified dog trainer and I have been training dogs for 19 years but not certified. That being said, I have alot of knowledge and many rescues ask for help (which is why I have dog #6).
That being said, a handler must always be in control of their dogs and their dogs actions. If you cannot answer any of the above questions I asked, then you should not be allowing your dog free in the park with others. Friends are friends until a vet bill ensues. Trust me.
What your friend can do is seek out a dog evaluator to find out what is going on with your dog. A behaviouralist can also be looked into, but I would start with an evaluator as it is less expensive. However, if the evaluator finds something that a behaviouralist can add insight to, then I would seek this out.
It is so difficult to give advice if we cannot actually see the behaviour. Indeed it can be a case of intolerance for puppies, but regardless, attacking is simply not acceptable. These puppies will get big and something can happen to this little guy in the future. Funny - people say dogs do not have a memory...I beg to differ.
Perhaps your friend can enroll in basic obedience classes. This will help considerably.