I'd just tell her. It's only right that SHE has a say so to what is happening to her body. I like how VegLife put it. I like what Casual Eagle said as well.
When I was in college (before I even knew all of what I was/ am and into), I had a friend whom I tickled a lot. And I mean A LOT!!!!!! And it wasn't short/ quick tickles either.
We stayed connected as we moved into adulthood until she made a pass at me when I spent the night at her house (as I often did in college). I wasn't ready for that. And I wish I had handled it better. Looking back, I'm 100% sure that my friendship with her was confusing to her. I correctly blame myself for that.
We reconnected and I told her over the phone that I had figured out that I was into tickling, and I don't think she understood that although I was into it, that didn't mean I was into her. Our friendship has been confusing and just odd ever since. I can't really explain our dynamic. We now live 20 minutes from each other, but we don't talk at all. She follows me on Instagram, I check in silently to see if she is ok too. I wish I could tell her that the person I was right out of college IS NOT the person I am now.
I know our stories are NOT the same. But, naïve, college-age me wishes I had not been so careless back then. 100% of it was out of ignorance, but that doesn't make it hurt any less.