A very similar thing happened to my friend a few years ago. I had just left them to head home as my horse was pratting around (there were about 3 or 4 of us all together) and they went for a gallop across a green. The horse just collapsed on top of her. It was later discovered it was a heart attack. She was fit and in work but the vet said these things do sometimes happen! Very sad for everyone!
Just heard this on the local news - thoughts are with her family and all my fingers & toes crossed for her to make a speedy recovery. Not sure if this happened at Blackwater Farm which is where several local pony clubs hold their camps.