Is it wrong to have sex with a married friend whose wife doesn't love him and refuses to sleep with him?

We are totally 'just friends,' there's nothing romantic about it. He loves his wife, but she recently told him she's not 'in love' with him. He wants to stay with his wife, and I have no interest in getting romantically involved with him. We both just really enjoy sex and don't have anyone else to do it with. I'm not into one night stands, and he doesn't want to hook up with someone who might be 'unsafe.' It seems like it could just be one more thing two friends enjoy doing once in a while without it being an issue. Yeah, I'm sure the wife would disagree, but we're all grown ups here, and I am absolutely no threat to her.