What has she been cleaning her face with? The first thing I would do would be to make sure she is using a good cleanser, toner, and moisturizer. I would highly recommend Clinique mild liquid facial soap, along with their toner and moisturizer. She needs to be sure to use it both morning and night. See how that goes. If she is just getting a few small pimples and that does not help, then I would try some spot treatment with some over the counter things. That might be all she needs. I got switched to that as a teen, and it solved my breakout issues and I had very clear skin as a teen with very few breakouts. As an adult, I started using other supposedly "better" products as cleansers etc. instead of that "old fashioned Clinique I used for years" and I started having some breakouts. I attributed it to work stress, or just being lucky and having it hit later than my teen years. I did not think it related to the Clinique. After several years, I started using Clinique again and to my surprise the breakouts almost completely stopped. I would start there before going to more harsh methods or dermatologists.
I think that a lot of teens have breakouts caused not just by the hormonal changes, but also by the fact that they are not used to having to really clean their faces day and night, and also by the fact that there are a lot of cleansers and products out there that can be irritating. If it is on her forehead and no where else, it might even be an issue with the hairspray or hair product she is using.