A married man has been having an affair with another woman for five years and since two years the man had second nikkah with her

Question

Assalamoualaikum,

Please advise.

A married man has been having an affair with another woman for five years and since two years the man had second nikkah with the woman. When first wife came to know about second nikkah, husband agreed to give talaaq to second wife. He then told the first wife that he has already given talaaq to second wife by phone as the second wife was refusing to meet him and first wife in person for purpose of giving talaaq. However, the first wife is asking husband to send a signed talaaq by post as she is not believing the husband . Please advise whether if husband sends a signed talaaq by post will it be valid as the signed talaaq may not be read by the second wife.

Jazaak Allah