No, you do not have to accept it. It doesn't matter if he had a "legitimate" excuse: employers do not have to let employees miss work unless they had and used PTO (e.g. sick days) to cover the absences, or unless they were eligible for (and the company larger enough to be covered by) FMLA leave and *at the time* the employee applied for an validly used it. Otherwise, an employer may terminate an employee for excessive absence, regardless of the reasons for absence; and in particular, may make the decision based on what is known at the time, and is not required to revisit that decision a half-year later.