Open the lines of communication! You need to ask the parents what has happened. Tell them that you are concerned and want to help the situation, did something happen that you are unaware of? None of us like to think that our kids have done something that is deemed inappropriate or has hurt another child, but always a possibility. Most importantly, do NOT be defensive if she says it's in reaction to something that your son has done. Remain calm, and tell her that you are thankful for the feedback, that you had no idea, and will address the problem accordingly. Then go talk to you son calmly, not pointing fingers, but give him a chance to tell his side of the story. Then calmly tell him what the other boy told his parents, and check his reaction, because somewhere in the middle lies the truth. If the hiding is due to your son's behavior, then tell him that it was inappropriate & unacceptable & handle/reprimand appropriately. If it's not due to his behavior, then tell him to handle his friend with care & to come get you if it ever happens again, so you can help out.
The other thought is that perhaps the boy is going thru something & the parents don't know how to discuss it, are embarrassed, or are in denial, and the uncle is someone who can help. Often when people are embarrassed, they come off as angry, because they are angry, angry that someone has noticed what they perceive as a failing.
Good luck, remember to be kind & don't be on the defensive. Remember, there may be some behaviors, jokes, etc. that your family finds funny/appropriate, and others do not, for whatever reason.
I know all of this from personal experience. We just had an incident with our son, where his behavior was dreadful, but at the same time, he came home having learned a song that we were not thrilled with, but the other family thinks is fun....
Regarding the embarrassment/anger thing, both my son & husband lash out in anger when they are embarrassed. If someone gets hurt, my husband's family tends to admonish the injured person for not being careful, etc. It's someting I am constantly battling, always telling my husband & child to not be angry with me or anyone else for worrying about them & wanting them to be ok.
A little about me: 42 yr old mother of 6 year old boy, expecting a little girl in early October, and married to a wonderful man for almost 14 years.