It largely depends on how good a job you have done as a parent so far in teaching her to be a competent, independent person. If you have taught her well, giving her gradually more independence and trust, then there is absolutely no reason why she should not be able to get around Europe safely with a small group of people aged 18.
I have a 17-year-old DD too, and we live in a large city in central Europe. I'd be more worried about letting her go to London by herself, TBH, that always seems such a dangerous city to me, and then all those drunk people on Saturday nights, ewww, that always looks very dangerous. So you see, risk is largely a matter of perception. German parents are still often freaked out about letting their DC go to the UK because of BSE (although less now than a decade ago)!
One problem is that British teenagers do appear to be ridiculously molly-coddled by comparison with many other European nationalities; many German teenagers go to the USA for a whole school year aged 15 or 16, for instance. My own DD did a 3-month exchange to another country aged 15, so I know that she can cope well on her own. How much experience have you given your DD so far of looking after herself?
If they're planning to go to or through Germany, carrying pepper spray is semi-legal. The trick is that it's officially for warding off animal attacks and has to be labelled for that use, but if you happen to have a spray with you and get attacked by a person, you are allowed then to use the spray to defend yourself. Bonkers reasoning, but in short, they can get Pfefferspray from hardware and security shops etc (although they're currently all sold out for obvious reasons). I don't know about the legal situation of pepper spray in any other countries, you'd have to check. However, my DD goes around our city day and night and has never encountered any problems or hassle. What happened in Cologne etc was really NOT an everyday occurrence.