SUBREDDIT: r/relationships
TITLE: Should I [21F] tell this girl [20s] her boyfriend [23] is on Tinder?
POST: Okay so, a week ago I met this guy on Tinder, we flirted a bit and gave him my number. He just recently texted me this morning and I forgot had no idea it was him. Whenever I don't know a number, I usually search it on Facebook to see who it is and most of the time I get my answer. 

Well, I got my answer and more. From what I saw, he's got a girlfriend of 6 months. I feel absolutely terrible and I'm wanting to tell her but I'm a bit torn. Had a friend tell me to just not butt in and mind my own business.

I'm hoping to get some more feedback on what would be the right thing to do because I'm honestly on the verge of messaging her.
TL;DR: 
Met guy on Tinder, has a girlfriend and I'm not sure if it'd be right to tell her or not.