I hope you have called the owner, especially to know what you are allowed to do (vet) if things do go badly.
This just happened to me the other day (indirectly). Horse has Cushings and is laminitic, and got into a grain with 26% NSC. He was up and down, but otherwise OK. My directions were to handwalk him, give him our colic remedy (tea and whiskey basically), give him banamine if he got worse/no change, and call the vet to talk and to get on standby. Vet was much more concerned about his feet since this is a major issue with him. He was standing in snow and his feet were iced whenever possible/not in snow. In the morning he got bute (for his feet) and continue icing/snow. He got a few handfuls of hay, but no real dinner, syringed mineral oil, very small (hay only) breakfast, then tonight my instructions were to mash his grain very well and give him a small amount. (Vet ok'd normal feeding, but I am being paranoid since I am not there myself and he was still a little "off" in the morning.)
He never had a real "colic" and vet thought maybe he was lying down because of his feet, but his feet (SO LUCKY) seem fine, and I think if they were sore he would lie down and stay down. So I think we were dealing with a mild colic, and got VERY lucky with his feet (he is EXTREMELY sensitive to his diet).
The pony got out at the same time, probably did not get any grain. She seems ok and just got a small dinner (no grain).
So hopefully my little story will be helpful to you. I would not medicate unless the horse is showing symptoms (as said, call and talk to a vet, they can advise- and talk to the owner!) but a home remedy like I said won't hurt, though again, I would probably wait. Might try mineral oil though. Try and figure out which ate more (watch them both of course!) My main concern in this situation would probably be colic. Just give them a very small amount of hay to eat, water. Monitor input/output of hay/water, and check them frequently.
I definitely agree with Sharpie, that things are USUALLY OK, but if they aren't it can be very serious.
Don't check the legs check the hooves.