The difference between a practical joke in good taste and bad taste is that the good taste jokes are completely reversible, while the bad ones arent. Unless you somehow make it blatantly obvious that he isnt actually a sexual predator there may be some irreparable damage done here. Whether or not there is, or whether or not he breaks off the freindship because of it is a different question, but there is definitely room here to possibly cross that line, be careful if you value the friendship.
But otherwise, great idea
