If she's not safe in Europe I don't know what other continents she can go to! The worst that could happen and probably won't is someone steals something of hers, which to me doesn't make her unsafe. Just temporarily less wealthy. I've met loads of solo female travelers in Europe, all very much alive. Common sense is all she'll need.
Generally speaking Eu is safe.
Bigger cities have more issues than smaller, certain countries more than others.
Balkan region and east europe is safer than western.
In southern regions a girl would be probably 'annoyed' by locals approaching her, but it's nothing dangerous if she firmly but kindly refuse to talk.
I agree with the other answers - it is all about being sensible and using your common sense - if your daughter does this she should be as safe alone in Europe as anywhere. As long as she keeps to places where there are people about and doesn't flaunt her valuables and as long as she is sensible when communicating with strangers she meets she should be fine.