I saw this movie on video circa 1986. I think it's French but it could also be US American. The protagonist is a likable, non-threatening guy like Pierre Richard or Gene Wilder.
The girlfriend makes a nude photo of herself per self-timer for her affair (she's seen pushing a button on the camera and then rushing to her bed to pose topless) and her boyfriend finds that photo (maybe a Polaroid) and probably a letter and he goes to his best friend (our protagonist) and asks him to find out who the other guy is. Of course what he doesn't now is that his best friend is actually that other guy.
I think the location is a skiing resort but this could be from another movie ...
I think the main story takes place in a skiing resort and I think a skiing chairlift plays a role or was on the video cover. Or maybe that was another movie. Also there's a supporting role of a guy who breaks his arms or legs and walks around with his arms in plaster (of Paris) and a pair of skis on his back, resulting in slapstick humor (that could also be that other movie).
That other movie that may interfere with my memories is Just the Way You Are (1984) starring Kristy McNichol. Synopsis: "Susan is a young, beautiful and successful flute player, but because of her physical handicap, a lame leg, she is having difficulties finding Mr. Right. While on tour in France, she decides to spend a few days on a ski resort wearing a fake cast around her lame leg to make sure her handicap goes unnoticed. Once arrived, she meets Peter, a handsome photographer."