Actually there was a horse (a baby) in barn I used to board with one shoe on. The reason was it was born with one leg longer, so the farrier put a correction shoe to even them up. Eventually the horse didn't need that shoe anymore.
As for this situation I don't quite get HOW the farrier put one shoe on only? Or did they nail it themselves? My guess would be the horse had both shoes on, but lost one. If that's the case I'd mention it to the owners and either call farrier ASAP or pull the shoe off till farrier can come.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."