Yup, what 3and2 said. Depends on context. If it is meaningful to the conversation, yes. Generally, you wouldn't bring it up unsolicited unless the offer in hand has a time stamp and, therefore, relevant to the discussion. I wouldn't discuss details, again, unless very relevant to the conversation. Example would be if current conversation offers and it is less than what school A offered and you are prepared to have a serious negotiation where you know what your objective is. Also as 3and2 said, if the coach he is talking to is genuinely interested in the player, he may very well ask what other schools he is talking to.
Also know that the coaching fraternity tends to be a small world and they often talk to each other. Be respectful of confidential discussions and don't try to BS, exaggerate or bluff.