What lie could you say about him that's more damning than the truth?

That one is easier to answer than some might think, just make them hoist themselves on their own petard by lying about virtues because the truth mean dispelling those virtues which they will without thinking it through.

"Person A is extremely kind and generous, they donate to several charities and perform lots of work for the community."

Person A: "No I don't."

🙄

The US version of The Office does this at one point when Dwight and Jim are trying to frame each other to get the others Xmas bonus. Dwight gets control of Jim's credit card after Jim intentionally loudly speaks his card details for something however Dwight uses it to buy a $200 bouquet of flowers for Jim's wife. So while Dwight still ended up spending Jim's money it happens in a way that makes it difficult for Jim to claim it was Dwight as it's something people would expect a husband to do first of all but also something nobody would expect Dwight to do as it's too nice a gesture. If Jim admits the truth he's stating he didn't buy his wife the flowers as well as confirming he intentionally gave Dwight the card details, so he's hoist by his own petard.