Lisa never went on vacation with guy, but she used to do regular business trips with a coworker.
"Ryan" was the team lead so he always got to pick the people that went on the trips with him and after the first time he fucked her he always chose her. Their company always paid for separate rooms for everyone, but she always stayed with him in his room, so it wasn't a secret to the rest of their coworkers.
He didn't know I knew he was fucking her. She said he really liked fucking another man's wife behind his back, so she let him think I was clueless. The upside was I got to read his text messages telling her how much he missed seeing her big tits bounce while he fucked her and how much he loved the look on her face when he came in her. The downside is when she called I had to play dumb because he was in the room with her and unlike a lot of other guys, he never sent me pics.
One time she called and I could tell something was going on, but I said nothing. Later she texted me that while she was on the phone with me he had her bent over the bed and was slowly grinding his cock in and out of her, torturing her into wanting to moan in pleasure while she tried to keep up the illusion that I didn't know.
This went on every other month for about three years.