This is a Hypothetical question! - If a person sends their bitcoin to the cold wallet of a friend does this effectively destroy the CGT (capital gains tax) trail back to when they purchased it? - ie: If the friend sells it in a month is the only CGT liability for that 4 weeks?.
It will depend on whether your friend can prove all of this legally (you will have to draw up a sales deed and stock up on evidence).
The burden of proof will fall on the shoulders of the last buyer of bitcoin.

Most importantly how do the tax people know that the BTC changed ownership from the original owner to the friend and how do they know about the taxable event that relates to the original owners Capital gain?
They won't "run after you and look for information", because they will pass new laws in such a way that you yourself will run to them and reveal all the information about your assets, voluntarily-forcibly.
- How do the Tax people see this?...if they can at all?....
They're "blind" until you tell them everything yourself.
These guys are in a "different" financial system.
I cant see that they can which would make this a tax loophole?
Therefore, first of all, before they start pulling taxes from you, these guys will force you to declare information about your wallets and crypto assets.