Seriously, Wistrash is getting praise for not defaming someone? "Oh look, he doesn't go out of his way to slander her! How meritorious!" Plus, the plan is insane. Eustace might be safe just spreading rumors, but him confirming them publicly would put him on very dangerous ground. It would be a grave insult to both monarchs when it is proven false, and he'd have no way to offer any kind of evidence for why he believes it either. They weren't ever married, just engaged, so it's not like he can point at the lack of children as evidence. So what is he going to do when someone asks the very reasonable question "how do you know she's infertile?" You'd expect a doctor to have reported it, so then he'd have to produce the doctor. So he either has to keep building up on his lies, digging a deeper hole for himself with each lie, possibly breaking multiple laws, or come clean that he was jealous. It's a dumb lie that could ruin his life, and it wouldn't benefit him in any way. Not going along with the plan isn't evidence of being a decent person, it's having a functioning survival instinct.