How do you help someone you barely know?

I'm on a field trip with a college group right now, right before all of our freshmen year. There is around 50 of us, and we are spending 10 days in London.

Today is day 9. I did not know anyone here before this trip.

There is a girl who I'm 90% sure is hurting herself. (Clocked it because been there, done that, 1.5 years clean and doing MUCH better). I wasn't going to say anything, but I swear there is a new mark that is only a scab, and some aren't even scarred over yet.

She is pretty quiet, but overall SEEMS fine. She isn't necessarily avoiding people, but today I was talking to her and she would just nod and shake her head. Not with a sad face or anything, just didn't feel like talking. I breifly asked if she is alright and she said yeah kind of contently.

There is a chance she is fine, just quiet and content, and the marks are for something else, but I doubt it. We are in London tomorrow, leave the next day, and we share a floor in college so I will see her again, just not for another 2 weeks. Any advice is appreciated. Right now I'm just trying to be friendly.