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.
"You win some, you lose some, you wreck some." -- Dale Earnhardt