Many foundered horses can be managed so they can again become useful, but many also end up chronically lame and some are affected to the degree they must be humanely destroyed. Some of those big, fat, easy keepers, just cannot be on pasture. Some will founder just eating the weeds and grass they can reach through the corral fences. The other thing to remember is - all things in moderation. Too much grain is not good.
Oats are the best when feeding grain.
If a horse is hot, cool him out and water him gradually. Just think of your horse before you think of yourself.