I say you are not helping anybody by keeping a secret If they are your friends, they should understand that it is in the interest of the child's well being. If they can't understand that, then maybe you weren't that great of friends. I knew someone whose sister knew something similar about one of her kids. The sister chose to be the kid's friend rather than to let the parent know. It caused quite a rift. Eventually, the sister apologized for making a bad decision to withhold information that was important. She originally felt she was protecting the child, when in fact she was enabling the bad behavior. Your call, but if it were me, I'd say something.
A good corollary to this is the cheater spouse scenario. Most of the time, the person with the dilemma did not seek out or ask for the information, but once in possession, has to decide what to do with it. Always a tough call, but then, doing the "right" thing is never (in my experience) the easy thing.
"Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive" -- C.S. Lewis
If the only way to combat "global warming" was to lower taxes, we would never hear of the issue again. - Anonymous
"We will not solve our problems by electing the right people. We will only solve our problem by making
it politically profitable for the wrong people to do the right thing" -Milton Friedman