I shudder to tell you that once a client, a joint owner on Mom's account, came into the branch with Mom's statement and demanded to know where the money pertaining to a large withdrawal had gone. Mom had signed the withdrawal. The answer was it went into a new account that daughter did not know about, until my helpful banker provided her with a copy of the offsetting credit. The new account had a different joint owner, the granddaughter, and all of that hit the fan. Sounds like some sort of similar situation here. If the joint owner had a copy of the cashier's check the most likely next request would be knowing where that check was negotiated. The fact is that this issue is between the accountholders.
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"You win some, you lose some, you wreck some." -- Dale Earnhardt