What's actually the purpose for?

Do you want to buy Bitcoin without submitting KYC? just use no KYC P2P or DEX, no need to ask your friend to buy behalf from him.

On the other hand, I don't understand why your friend need to hand his coins to you because he has no way to access his coins anymore, what he can is ask or force you to move the coins.

When it comes to tax, your friend will not have tax obligations since he didn't sell his coins, capital gains tax occur after you exchange coins to other assets e.g. coins, fiat etc.