Sweep it directly to your own cold storage wallet immediately. Do not import the key into any online software or website. Use a trusted, open-source wallet like Electrum in an airgapped, offline environment to sign the transaction. Broadcast it quickly.
That is not the issue here.
Those "
puzzle" private keys have limited range, #71 specifically is easy enough to be solved in mere seconds once its public key is exposed.
The moment you broadcast the transaction to the network, signed offline or not, its public key will be revealed to those thousand "
puzzle-stealers" who watch it.
And given the info above, it'll just be a game of who's transaction that spends the same input that'll get into a block.
It may take a lot of quick unconfirmed txn replacements that could eventually spend the majority of the amount as fee.
So even if he somehow wins that, the end result wont be good.