If he will have to convert the USDT to bitcoin and then use a mixer to mix bitcoin, he will also have to convert the bitcoin back to USDT. That is a long process when he can just convert the USDT to monero and convert it back to monero which is a shorter process and the fee would be less than convert to bitcoin and using s mixer. The only time I prefer to use or involve bitcoin is when I want to mix bitcoin.
How do i convert my USDT to XMR (i have them on trust wallet) and then where i mix the XMR ?
I did not see monero/USDT trading pair on Bisq. You can first convert the USDT to bitcoin on an instant exchange, then convert the bitcoin to monero on bisq.network, convert the monero back to bitcoin and exchange the bitcoin back to USDT on an instant exchange.
To do that, you can use
https://exch.cx/ for the instant exchange
Use
https://bisq.network for the decentralized exchange.
Or to make it simple, use Tor on
https://exch.cx/ to convert it directly. But
https://exch.cx/is an instant exchange.