I've almost always traveled alone, and young people in particular can meet up with one another on trains or in youth hostels or B&B's.
In fact, I know from experience that traveling alone is much more pleasant and much more likely to bring you in contact with the locals than traveling with another American who doesn't like the same things you do or who acts like an ugly American. If the friend is backing out for reasons other than lack of money, it's likely that she's not a good traveler and is scared of being away from what is familiar, the type of traveler who makes a beeline for McDonald's in any foreign city.
The usual safety tips apply, the ones you would follow if you were in an unfamiliar American city. Rick Steves' guidebooks have good sections on the scams that target travelers. Keep your valuables (cards, passport, plane ticket receipts, rail passes) in a neck pouch and just small amounts of money in your purse, and keep photocopies of your passport and credit cards in your luggage.
Some travelers seem to want to call home all the time. This is just plain stupid unless you're made of money, because roaming charges, especially data roaming charges, will eat up your bank account. Barring life-and-death emergencies, there's nothing that can't be handled by e-mail.
Even the "arrived safely" message can be done by e-mail, since it will mostly likely be the middle of the night, U.S. time. For local calls in Europe, get an unlocked GSM or 3G phone and buy new SIM cards in each country. I save my budget by turning off cellular data when I'm overseas.