Hmmm... I don't know.
I wasn't charged at all when my horse coliced. He was hand-walked until I got there and took over (probably about 30 - 40mins?), she stayed with me, assisted the Vet (Vet wanted to know every last detail about his day, the hay she was feeding him etc....), stripped and double-bedded his stall as he was given a pretty hefty sedative so the Vet could tube him and then checked on him every 2 hours for the rest of the night, after I left.
Since you did cancel a lesson, I'd ask the boarder if she feels the $40 lesson fee would be fair.