Thank you 'trying' for coming to the aid of this sheep. It may be the owners responsibility but it would be inhumane to ignore an animal in need.
It sounds as if it perhaps already had something wrong with it, as although it was in an awkward position, it was not classically cowped ie upside down. Still, with it's backend higher than it's mouth, the gases would have built up in the gut, so the same prinicpal would apply as when a sheep is cowped. So first thing is to stand it up and hold onto it while it burps and gets it's bearings. If it is ok it should be alright to let it go, although the owner might prefer to give it a once-over himself.
Your ewe appears to have hurt a leg and to be in poor shape. Check it hasn't actually broken a leg, but even so stand it every couple of hours to burp and pee. You are doing what you can, with some food and water until you can contact the owner, who will hopefully give an immediate antibiotic suitable for pneumonia before seeing what else can be done - an animal which has lain all night will be at severe risk of pneumonia.
Just to be on the safe side, as soon as the sheep has been collected, burn the straw or bedding it was lying on and disinfect the horse box. Also do not handle your own animals until you have had the chance to clean up.
I hope she is ok and that your neighbour takes proper care of her, even if that involves having her put down.