If you and your s/o have been arguing a lot , rarely have sex anymore and find underwear in his car would you assume he is cheating ?

My Sister’s boyfriend did a photo shoot for their friend and afterwards my sister found the friends underwear in the car . ( my sister was at work and did not go to the photo shoot ) They said she changed in there but Her boyfriend went out of the car while she changed . My sister doesn’t know what to believe , she is so confused and crying