How unfortunate for the poor lady, but fortunate for you to have your camera ready to get the "perfect moment" shot. You got a great background setting too. The only suggestion I would offer, if you're into photo editing, is to lighten the subject's face a bit to capture the reaction expression. I'm curious as to whether it's shock or laughter.
It's a good subject, but I find the lighting and/or background to kind of take away from the picture. I realize it was one of those hard to plan shots, but it'd be nice with a plainer background.