Well, I've been in a few gigh school relationships...
the first one lasted three months, the second lasted eleven months, and the third lasted about three and a half months. but I have friends who have a new boyfriend every week. my relationships have lasted longer than most, basically.
The best you can do is be the best friend you can to her. Always think about what you can do to make her happy and help her out. Chances are, she and this guy will break up after a while, and if you're a good enough friend she'll turn to you for comfort.
Oh, and I should mention that it's important to ALWAYS LOOK IN HER EYES while you're talking to her, not just generally at her face. Having one's eyes gazed into can spark romantic feelings even if one doesn't realize it at the time!
Good luck.