I don't see how using block explorer websites is going to help here. Is he going to ask the seller to identify the specific UTXOs they are going to trade with prior to the trade? And then what happens if the seller uses other UTXOs instead? He's not going to be able to reverse the trade because he doesn't like where those UTXOs come from.

I would also say that I have traded bitcoin peer to peer, a lot, and never once cared in the slightest where the bitcoin I am buying comes from, just as I do not care in the slightest where the cash I receive comes from. I'm not going to spend my cash anywhere that looks at the serial numbers and decides to refuse my cash because it was once used to commit a crime, and I'm not going to spend my bitcoin anywhere that looks at the on chain history of that bitcoin and decides to refuse it.

The solution to your friend's problem is not to start attempting to investigate specific UTXOs before he has even bought them. The solution is to identify why he wants to do this and which services he is using which are placing artificial and arbitrary restrictions on his coins, and use alternatives instead.