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Normally the bank would be responsible since he was not on the account but you are charged with securing the checks and since he is a partner they are not responsible. Here I still disagree that law enforcement will get involved. While you may have a civil case, I do not see them charging him with anything criminal although he may mot be aware and you never know when LE will get involved and when they will not since it is an individual decision.
Here I still say the best way out is to pay him something and be done with it. Here he amy sign just in return for forgiveness of what he has taken.