Bought a Eurail pass because friends had told horror stories of bad experiences hitchhiking. Used "Europe on $5 a Day" as a guide (this was a long time ago!) It was great. I met people in various places, stayed with relatives and family friends in a couple of cities, in inexpensive hotels or bed and breakfasts the rest of the time.
Because I was traveling solo, I was able to do what I wanted when I wanted. It was great. I learned a lot, met very interesting people, had some great experiences.
If your daughter has good common sense, she should be fine on her own. There were a few sticky situations, but nothing that I couldn't handle.
Years later when I did a trip with someone I knew, it was awful. She disapproved of nearly everything that I did (and vice versa). The worst part is that, unknown to me, she was taking medications that should never be taken with booze.
In addition to the Lonely Planet books mentioned above, Rick Steves' European travel skills programs offer lots of good information.
http://www.ricksteves.com/tvr/season7/episode-info/european-travel-skills.htm
Best of luck to your daughter.
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