Guy Sleeps With Girl Then Tells Her Boyfriend

A guy slept with a girl, but then she told him the next day that she has a boyfriend.  So the guy gave the boyfriend the heads up, if you'll pardon the pun.

The guy took to Reddit to tell his story, but it goes like this - he slept with some chica who told him the next morning that she has a boyfriend.  The guy decided to let the boyfriend in on what happened by taping a note to the underside of the toilet seat.  The chica isn't going to see it because girls don't lift the toilet seat, but the boyfriend would see it.

So the question is, did he do the right thing?