Honestly, I'm usually one of the more skeptical people here regarding the "it wasn't a tickle" category, but he put all of her toes into his mouth and I am 95% sure he was licking them for at least a couple seconds.
So here's the thing. I don't think it was an EXTREMELY ticklish moment causing her to laugh like that, but I feel like it is entirely possible that it tickled a little bit, or at the very least it felt weird. I'm sure her feet are a little ticklish.
So a weird sensation on her bare toes in combination with having her bare foot shoved in some guys mouth on a live audience stage, and the general ridiculousness of the whole situation could have led to her reaction. It is also possible she played up the reaction a bit.