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He can be terminated on the basis that the urination outside was an "intervening cause" for his termination.
Being on FMLA doesn't insulate one from termination for offenses not actually related to FMLA absences. Certainly, there is an issue with the bathroom not being where it should be, but that is just a basis for a complaint to the DOL against the employer for not complying with the rule. That is not a basis for allowing a person to engage in what could be classified as misconduct.
So, the only way that your husband could argue that the employer here is engaged in discrimination is if he can show some evidence demonstrating that other people have committed more serious "offenses" and were not terminated. Then it will appear that the employer is intentionally overreacting due to his disability or because he was using FMLA.
He would make that argument to the EEOC (and the DOL for FMLA use discrimination).
So, they can terminate him and hold the position that it was an "intervening cause" that would justify the termination, and he can then file the complaints with the EEOC and DOL. Regrettably, this is a really close call and it will depend greatly on what other offenses can be pointed to where they chose to keep the employee.