Its scary if you dont know anyone full stop imo. But its a bit irrational if the worry is what people think about the age difference.
I would say if they are her real friends then they will be really happy for her that shes happy with you.
And tbh, 9 years is a normal difference for men and women its just usually been the man older. But now days its, imo, not that big a deal to people for women to be older than a guy. End of the day, does it bother you? If not, why should what others think bother you? What REALLY matters is what you and her think of it.
Just relax, dont get hammered and make an **** out of yourself, show her friends what a courteous, polite and nice guy you are and you'll be fine.
