Options seem to be:
1. Call child protective services, who won't do diddly about it unless the kid is being physically abused;
2. Verbally confront adult about it...99.9% odds are that it won't change anything about their behavior, it will just piss them off and just make whatever job/payment arrangements there are in place more difficult. they will probably complain to your supervisor (or BBB if you are self-employed) with some made-up BS about your work, etc.;
3. Physically confront adult by giving them the hardest bitchslap you can possibly muster. Although this is really what is needed in this situation, again, they are likely to call cops and the person arrested will be you, no matter how much in the right you will have been.
4. Forget you heard it and feel sorry for the kid that is being raised by such a piece of shit and feel even worse that as a result of their parent, the kid will end up in juvy one day, then jail, then have kids of their own and repeat the cycle again....which wil lead you to feel sorry for our society as a whole.
5. Decide that the kid probably is a fucking retard and go on your merry way.
EDIT: forgot this one...
6. Realize that growing up in such a tough environment will allow the kid to become a Navy SEAL, who will then have issues b/c of that upbringing, so instead of protecting the country, he will devote his time to posting on internet message boards.