Actually, that’s not what the OP (in the quote you provided) says.
She says her arena is “set about 15 feet off that fence from my neighbor’s house.”
That means the arena is 15’ away from the property line/fence (at least, she thinks; since the neighbor moved the marker, who knows?). She does not say where the neighbor’s house is located in relation to the property line.
But if his house was right on the fence line - if it, was in fact, just 15’ feet from her arena, let’s say - then why would he need to drive his car to the fence, sit in it, and watch her give lessons when he could do all that from the comfort of his own home, which in your interpretation is also just 15’ from the arena?