It's even better with a friend like that...long time friend, very close but nothing has ever happened...
Seeing up her skirt was probably a bigger turn on for you than seeing up 100s of skirts of people you don't know. Especially as she quickly became comfortable with it and wanted to make the most out of it.
I have a close friend, and I accidentally saw her belly button the other day, and I about came then and there. That was the most skin I had ever seen on her, and being it was unintentional made it even better. Yet I will see down inside oversized bras of random women at the store, and I couldn't care less.
I remember being on a family trip with my wife's sister. She was lying on her bed one morning reading. Pj pants on and a pj top with her midriff barely showing and her braless nipples point towards the ceiling. Just a typical woman reading in bed in pjs, but it was one of the most exciting moments of my life around her. If it were anyone else, I wouldn't have even noticed.