Re: In a sticky situation ...
Hmmm interesting. The easy answer is to reassure her and convince her that it's been a long time so it doesn't matter. But that could make it worse.
I'd go with "whatever you put your attention on, you give it power."
This sounds like something she has to work through in her own mind. Any interference from you (Ex: "Don't worry. It was a long time ago and I don't have feelings for her anymore etc.") could just add fuel to the fire. I think if you just ignore it and brush it off you'll be fine. She'll accept it in her own time. Just keep changing the subject when she brings it up and make sure you are spreading good vibes when you are together and have fun.
I've done this myself with most issues that women have with me, such as if we were friends for a long time or my exes friend, and it's worked fine for me. Hope this helps and good luck.
"All things are subject to interpretation. Whichever interpretation prevails at a given time is a function of power and not truth."