Pruce had nothing better to do than to interact with these two strange creatures while waiting for Ivee to get back with Tokk, and even if they were nuts- goddess only knew what their reasons for it would be, it could've been something traumatic that they overwrote in their memories, for all Pruce knew, so he'd be happy to play along, if it made them happy. He liked Link, after all, kid was a joy to buy and sell with, and even if he was nuts, he regularly kept the village safer than anyone ever had before, regularly wiping out the monsters that lurked in the trees along the path leading into town.
So, he figured, he may as well insert himself into this conversation. Nuts or not, they seemed like good kids, and he knew Link lived in town, even though he was gone frequently, and this woman was apparently staying with him, and as far as anyone in town would be concerned, that ultimately meant Link and Zelda were one of them. Just... possibly their town kooks.
"Kid, there is no way in the world to not be fazed by that," he said. "My wife and I wanted Ivee and it still knocked me on my rear when she told me she was in the family way."