In that case he won't be held liable as he had tried as any reasonable person would. If the company did not reply, it is their headache.
If he is ready to pay it would be better and he can ask for a settlement amount from the company as they have been at fault here.
The amount of salary should be the exactly what he received. The company may ask for interest on the money too, but again as they were informed through post, you can fight over the same and win it too. Engage a lawyer for the best advose in this regard when you send a notice or transact with them. He will be able to put words in a way that you are safe from any further hooks that the company may throw at you if the intentions are not bona fide.