I don't believe that her friends are necessarily jealous. I have had friends be concerned for my well-being as I am going on my first solo trip to Paris in May also. As I've explained to them, people have been travelling to Europe alone for many years, some right out of high school. A person just has to use good judgement no matter where they are.

I just took a solo trip to Rome and I am/was in a situation very similar to yours. All I can tell you is to really enjoy taking the opportunity to do things for yourself, remain open to interactions with others without opening yourself up to opportunists. If you are thirty-something you probably know how that works. ;)
Good luck, have fun, bon voyage!