I guess that you " left coast softies" have forgotten how much water horses, cattle, llamas and even a lot of chickens can drink. LOL If you just leave the tap open to a trickle and the line is deep enough, you will not have freezing problems. That will save a lot of electricity, and the dollars for it, over the years. I don't know about CPVC or other plastics, but PVC can crack with ice in it. Copper and galvanized pipe will not leak after freezing but the fittings, particularly valves, will.
Our kids 160 year old farmhouse has water pipes that run from the bath to the kitchen through an unheated crawl. After I spent some glorious hours thawing pipes with heat tape around them, a combination of letting the water trickle and some insulation on the ground solved the problem ever since.
I would run copper at 42" deep from Mary's house to a frost-proof/insulated box inside the barn with a frostproof sill cock to fill the horse trough, or other vessel. Make the box top section removeable for thawing, should you have to.
If Mary just wants the water to top off the swimming pool, then put a stop valve and a drain valve in the house and drain the line in the winter. Depending on local codes and common sense, she should have a check valve in place to stop any chance of water from the barn line siphoning back into the house, for either kind of usage.