Europe in general is pretty safe, but some countries, and indeed towns and cities, are riskier than others. The biggest risk is theft - pickpockets and bag thieves. From repeated personal experience, such thefts are rarely violent events. Instead, things vanish so deftly it’s as if the Enterprise beamed them up while you weren’t looking.
It’s entirely possible to wander naively through Europe with your head in the clouds and get away with it. (I’ve done it, and I’m on full-time autopilot!) It’s also entirely possible to be a suspicious cynical security maniac, and STILL have some artful dodger vanish your moneybelt without you noticing. (I’ve done that too, in a country sufficiently alien for me to turn my autopilot off. These guys are good, they do it for a living.) But you’re much less likely to be selected as a target if you give out the impression of a suspicious cynic rather than a naive tourist, or at least look like you’re paying attention.
The best thing to do is give yourself a lot of contingency options. Don’t carry around large amounts of cash; use debit cards and/or credit cards. One main card, one spare, and don’t keep them together. Make sure they work in Europe, especially in ATMs, BEFORE you arrive - check with the bank. Scan your important documents - tickets, passport, insurance, and email them to a web-based email that you can access anywhere. Same with important phone numbers.
The worst things to lose are: Passport, credit/debit cards, holiday photos. A stolen passport is not a total disaster - an American consulate should give you a document to get you back into the USA, and once in Europe you can travel around without a passport. (Except to the UK, we’re a bit backward.) Loss of one card isn’t a disaster either - cancel it and use the spare. If they run off different bank accounts, make sure you have money in both. With internet banking, you can even shift your money around from an Internet cafe, and they are EVERYWHERE.
Losing holiday photos on the other hand will gnaw at your soul for the rest of your life, and cameras are a favourite thieves’ target. Photoshops everywhere will let you burn your memory cards to CD. Do it often.
All of which is a long way of saying, if she makes a few preparations before she arrives, takes precautions and uses her common sense, she should be fine and have a fantastic time.