On the lowest level, she could be spreading diseases. Otherwise, on the natural level, she's learning pernicious habits that will be hard to break in later life. It will be much harder for her to establish a hold a monogamous relationship in later life, which could harm a future spouse and children. Currently, she's also setting a bad example for everyone who knows what she's doing. She's degrading every man that she fornicates with (as well as herself). She's also breaking her parents' hearts, if they're decent parents.
At the highest level, she's seriously jeopardizing her salvation and that of others. Not that we should care about her or anything. To hell with her, right?
Assuming she's a grownup, her actions and their consequences are her responsibility, no one else's.
Perfectly put.