It's my understanding that he can simply spend those coins to another 1x legacy address to remove taint. I believe a 3x address spending back to a 1xx address requires a signature in the blockchain, so when segwit rules stop being enforced by miners, the received transaction is still valid even though it came from a segwit address. I remember reading from the Trilema logs but not entirely clear if this is true in practice.
RE: Secrets of Bitcoin’s Dystopian Valuation Model