Instead of going the life insurance route, every year someone can pay someone else (it’s now) a $15k “gift”, tax free. And once it is given, it is non-refundable.
Even if his wife catches him and decides to nuke him in a divorce, that money remains his mistresses.
I would hazard to guess that as soon as she proposed such a deal, he would leave skid marks getting out of there, because he never had any intent, despite sweet words, of his mistress getting anything in the first place.
And sure as a gun, as soon as he dropped his first mistress, he would be looking for another gullible doughnut.
So first and foremost, his current mistress should collect enough evidence to prove they had an affair.
I was going to suggest jewelry or bullion. These days he could buy her Bitcoin. Good luck to the widow tracking that stuff down.