Arthur,
Do you have a Buyer Broker Agreement signed with your client? If so thats great!! If not your client can sign one with the other agent and it's a done deal for you. It may be shady on the other agent, but if you haven't signed anything with your customer (because without a BBA he/she is a customer, not a client)then the other agent is in the clear. If you have that agreement signed and your Client is willing to write a statement agreeing to the fact, then you might have a case for your Association.