I think it depends on the circumstances. For instance, if the woman was trying to get pregnant, I'd definitely tell her. If the woman herself was having an affair, I probably wouldn't. If they already have children and you think it's better for the kids if they stay together, I probably wouldn't tell. If they have kids and he was abusive, I'd probably tell. If they have an open marriage, I wouldn't say anything.
If my spouse was totally opposed to telling her, I'd think long and hard before doing so.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13