glad you checked with the owner about feeding,its a very sore point with horse owners.
You would be surprised waht gets launched into fields by people feeling sorry for horses, ie grass clippings, bags of apples etc. these can do more harm than good and can lead to mega vets bills and worse.
Believe it or not to much grass can also cause certain horses problems.
( google laminitis when you have a minute)
It could be this mare is being fed hardfeed/suplemants but you can only do so much of this and the horse will demolish it in minutes ! so you may well not see this happening (ie early morning / late evening)