Some kind of meaningful reward for the babysitter for sure, but I would also make a point that if the babysitter ever feels there's something "not right" like this again to call 911 and you'll cover it, right or wrong.

Around here it's a $500 fee for an ambulance call, and I suspect insurance (if you have it) applies a "20-20 hindsight" rule: they may not pay for the ambulance if it turns out there wasn't a problem after all. A reliable babysitter should be told to just call 911, and if it turns out it's wrong and costs money, that's OK.