I doubt somebody would get arrested simply for using coinjoin. If the user was innocent, but they were arrested anyways that would show they couldn’t trace coinjoins, they would just be accusing somebody based on wrong assumptions based on who they participated in a coinjoin with.
There are countries that are against the use of protocols that makes transaction too complex and untraceable due to their anti money laundering fight, they dislike when people are too private on the chain, engaging with such can lead to questions and investigations but from what OP saying, this happen six year ago, there was KYC and Aml around but it's not as tough as it's today and beside, who comes knocking on your door for using coinjoin if you are not connected to any exchange that was able identify him, the purpose of coinjoin was broken somewhere.
Wasabi coinjoins are easy to identify on the blockchain. Exchanges might see that you used Wasabi and it might lead to some questioning and freezing of your funds, but it would be very difficult to determine that you specifically are involved in criminal activity. They might arrive at that conclusion if you made a mistake when transacting, like spending anonymized funds with non-private coins.
Using Wasabi alone isn't enough to get him lock up, an output can be questionable but if he decide not to show his input there is no way they will ever know but like I said, the guy must have made a very simple mistake, there was no exchanges that were deeply forensic with coinjoin transaction, even mixers were everywhere doing there business and hardly you see reports about them until last few years. It's either this is one of the story that never happened or the guy did not properly use coinjoin well and must have spent a label profile coin.
