I saw my friend's girlfriend on tinder?

Anonymous
I was swiping through tinder, and came upon my good friend's girlfriend of a year. I don't know what to make of it because I lived with him for a year, and know for a fact that he's been faithful to her. Of course I swiped right, but I carelessly forgot to screen shot it, and now I have no proof. Now I don't know what to do. I know that there's lots of people in college who go on tinder just to mess with people. Do I tell him or just let it go?
I saw my friend's girlfriend on tinder?
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