The excess distribution will be closed out to retained earnings that will create a negative balance. That will happen when the shareholder has used up stock basis.
Some others here may also suggest creating a shareholder loan, but I'd caution that this should be fully documented and make sure that is the shareholder's INTENT, including paying it back with interest that at least meets the current AFR.
Also talk to your client about not using the S corp as his or her personal checkbook.