Ideally, you would reorg his blocks out and make the network progress as it should.
Fabian has a [PR](https://github.com/bitcoin/bitcoin/pull/31117) that allows you to reorg them. However I couldn't get ckpool to work with it. I've been meaning to write a proxy myself for a while, but never find time ๐
It finally dawned on my why this works. Since they have diff 1 blocks in their chain if I ignore and find a fat work block, I become the chain with most PoW.
It didn't click for some time. Lol