I do not know if what he is trying to present legally in all countries or even will not expose you to the legal issue, but it is better to avoid buying such wallets, as it may be related to stealing things or hacking, and then you will be prosecuted as the culprit.
Claiming deductions that you are not entitled to, or reporting losses that you did not actually incur is a fraudulent claim and is considered tax evasion, and naturally, that's illegal.
I don't know about other countries but in the US, if you are audited you need to show transactions that prove that it was YOU who bought assets at the cost basis. I don't know if that is clear enough, not just that you have a record of those transactions but it was YOU who made them. Here is more on how cryptocurrency is taxed in the U.S. -
https://www.moneyknack.com/post/44/crypto-taxesThat's regarding taxes. But be aware of a common scam which this may be a variation of.
In this type of scam, fraudsters claim to have a wallet with a large amount of cryptocurrency that they are unable to access or use, and they offer to give the wallet to someone else who can use it. When the unsuspecting victim adds their own funds to the wallet (to pay for gas fees for example to withdraw other tokens), they are actually sending their money to the scammer, who then drains the wallet using bots or other automated means.