What a pessimist you are, beso ! She's not said " Give me money or I'll tell your wife if you don't" THAT would be blackmail. No, he's sent money of his own volition, a gift to keep her sweet, and hoping that she won't tell. Many a woman, finding out that the man is married, has threatened to tell the wife , perhaps in anger,perhaps hoping the man won't leave her, perhaps for some other reason or none.
Nothing will come of it as far as he's concerned. Many a rich man has given a substantial gift to his lover on parting. He's not complained of blackmail. Here the Crown Prosecution Service would not entertain a charge even if he was unwise enough to complain now, because there would be no reasonable prospect of conviction
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Here in the UK she'd pay no tax, as it's a gift.
I too don't know what American law is . It may be there's a gift tax. If not , then she has an explanation for the money. One which, incidentally, quite a lot of women uset. Few men are that lucky, that married women give them money ! Seems unfair to me !