Q. I have no trouble laying down my family's law when my son's friend acts up at our house, but should I tell his mom?
A. If you feel justified disciplining this kid while he's playing at your place, then you need to feel just as comfortable letting his mother know that you did. Junior's going to tell his mom anyway (not realizing that in ratting you out, he's dropping a dime on his own bad behavior), so the last thing you want is that she think you were hiding the fact that you threw the Bad Boy of Playdates in the pokey.
When his mom picks him up, take her aside and, without making it a big deal, tell her that when her son threw the toys/teased the baby/(insert naughtiness here), you made him apologize/sit in the corner/(insert punishment here). Assure her that it's the same penalty you mete out for your own kids.
If his mom has a problem with this, then there needn't be another playdate until Junior learns to conduct himself appropriately.