lots of people who look cute together break up. period. Looking cute together in public has never, and will never be an indicator of a happy, healthy stable relationship.
that being said, while any break-up is sad, it's probably good for her since she's very young and got into this relationship at an even younger age. do any of us still want to be dating the person we were 'in love' with at 17? *shudder* I know my judgement wasn't spot-on at that age.
most people don't marry the person they fall in love with as a teenager, so this is totally normal. hopefully they can remain friends and have learned wisdom from this relationship that they can bring to future relationships.