You're overreacting. Yes tinder is known as a "hookup" app. But not amongst girls. Girls see it as a game, and just laugh and tell each other the ridiculous things that guys send them. I know girls on there that have boyfriends. And if asked about it, they would probably simply say "oh i just use it for entertainment, i dont actually talk to guys on there. "
I'm not naive, i know girls can be dirty. But they are usually smart about cheating, and wouldnt do something so obvious as to put a profile intending to hookup on a public app where guys close to a geographic area are supposed to flip through and judge girls based on pictures if they were trying to sleep with those guys. Not to mention most of the guys on there are pathetic betas...not cheating with material.
If she wanted to fvck around on your friend, she will do so regardless of some stupid app that every college chick in the country uses, single or not. And she'd probably get away with it too.
Don't get your friend worked up about something that's nothing.