After serving a termination letter (after being discharged from service), the question of resignation does not arise. But it could have been considered had it been given before the domestic enquiry. That means, after failing to defend in the inquiry, he wants to have a settlement. If you are sympathetic, you can accept the resignation and delete the enquiry proceedings (already recorded) and relieve him. But I prefer to say that after having prolonged the matter to the inquiry, it is better to stick to the order of the enquiry committee.
Regards, Madhu.T.K