Ok, for some reason I'm not understanding the question... Is Y a code word for cooter or is it a restaraunt or do you mean the YMCA? I'm sorry I'm sure that sounds dumb. Lol. If you're speaking of cooter, yes I would definitely tell them!
Of course! You don't want the person to go on thinking that he or she is amazing at it when he or she doesn't know what's going on. Just make sure to be gentle about telling him or her; sex is a very sensitive topic.