For the girls, it's Kelly Clarkson's "Already Gone" (which is the one you're probably thinking about).
For the guys, it's Green River Ordinance's "On Your Own".
If that's also the scene where she throws the phone in the ocean, it could be Aaron Zigman's "It's Me & You Suite". The part played in the movie starts around 2 minutes in.
But if it's not him, it's probably a good chance that the song's from the score.
