LOL
I actually did once. Not a guy friend, but the mom of a girl I was chasing when I was 18. I used to hang at her house a great deal, but she has a boyfriend, and she wasn't willing to cheat on him yet. So when I cam over often times I would sit talking to her mom for hours, who looked like an older version of her daughter, and they were legitmately often times asked if they were sisters.
So one night I can't recall why she wasn't there, but when I showed up it was just her mom at home. I think the daughter ended up cancelling the plans through her mom (ie told her mom to tell me she couldnt make it). So she invited me in anyway to talk, and at one point just says "You know Art, she is never going to leave her bf to go otu with you." I was stunned, because it never occured to me that her mom even knew I was chasing her (talk about being clueless). I denied any interest, she was smiling at me, knowing I was full of shit. And somehow we got to the couch, just talking. Then she was stroking my arm while we were talking. And then it just led to kissing and the rest.