I did this once with my girlfriend. For my job at the time, I had to travel internationally for a week or two every month and always wondered what my girlfriend was up to while I was gone. I had a buddy who I asked to check in on her while I was gone during one extended trip. I had shared some of her naughty pics with him in the past so he was more than willing. It wasn't in the plan originally that he'd seduce her but we agreed that he should ask her out just to see how she would respond.
She worked a small store that almost never had customers so he had plenty of opportunity to meet her naturally. Anyways, he went there one day and made up some story about being new in the area. She helped him for nearly a half hour but he didn't buy anything. While he was leaving, he asked my girlfriend if she wanted coffee. She said she'd love coffee but she couldn't leave her shift. My buddy then left, got coffee down the street, and brought her back a cup with his phone number.
Later that day, my girlfriend texted him and thanked him for the coffee. He said it was no problem and asked if she had any recommendations for a bar to go to later that night. My girlfriend recommended a bar down the street from her apartment and my buddy said he'd maybe see her there later without explicitly calling it a date or putting her on the spot.
When he went to the bar later, she was already there and had a couple drinks already. She greeted him warmly and they sat next to each other and talked for about an hour. When they left, they started kissing and groping in the street. My girlfriend then led him to her apartment and they had sex. My buddy apologized about it afterwards but said she was incredible and he couldn't put the brakes on her even if he tried. I was jealous but thanked him for at least telling me what happened.
He hooked up with her a half dozen more times, always while I was on business trips. She never told me about it but she would text/sext him occasionally while I was home and he'd share her messages with me afterwards.