I may add,
It's open for him to decide what suits him in the circumstances -
I doubt his appointment letter (the contract agreement here) would contain a termination clause, in this case his resignation. In any case 'one month of notice' on either side is reasonable. He has not indicated whether only he was short paid/unpaid or everyone in the co. was paid/unpaid like this. If it is for everyone, he may have a case for himself. And what are the pluses in continuing the service with them ? How many years already served, what are his prospects outside ? Few suggestions, if prudent, may be considered -
1.He may, instead of withdrawing the resignation, issue another letter stating, non-payment/short payment being a breach, his resignation may be treated as due notice without prejudice to his rights in order to fulfilling the obligation of notice period if at all he wish to press for a resignation;
2.I hope, he still not received any communication on his resignation countering the action and therefore, the notice period has started ticking;
3. In order to sail smoothly, he should talk to his HoD/HR to ascertain the facts and to explain the circumstances (I believe he is otherwise satisfied in this co.) based on which he may decide on the next course of action;
4. If he has already an offer outside and when these fails and reached no return situation he should send a legal notice to secure his position.