Is it permissible to repay a debt to the second wife of the testator after his death, given the presence of the first wife and other heirs, or does this fall under the ruling of "no bequest to an heir"?

A debt transfers to the deceased's heirs after his death and is divided according to the prescribed legal shares; it is not exclusive to the second wife.

The deceased's will that the debt be paid to his second wife might be merely for the purpose of handing the debt over to her so that she can inform the heirs of it. Or, he might have intended to transfer ownership of the debt to her, and this is a bequest to an heir, which is not valid unless the other heirs agree to it.

You should clarify with the testator whether his intention was to transfer ownership of the debt to the second wife. Explain to him that this is a bequest forbidden by religious law and is not valid unless the heirs agree to it, or if his intention was merely to hand the money over to her.