"Mixing sites" are scams because you trust third parties with your data and with your coins
Stop spreading false information. Mixers are not scam. He can use a reputable mixer. It will have onion link and he should use Tor to access the onion link. I will not say that you do not know what you are saying because you know about this very well, so I will say you should stop lying.
Although some mixers may turn out to be scam, it is false to say mixers are scam because they centralized.
If you go down this road, github, Amazon, Google, Apple, Visa, Binance, Coinbase, are all scams because they are centralized.
Centralized services have flaws, but this is how the world works. There are many centralized services in the world which are legit and useful.
Some mixers had legal problems in the past, but mixers are not illegal by definition.