Your gut is right, its a massive red flag for regulators. Wallets that built-in mixers, like Wasabi or Samourai, have faced serious legal heat. It directly touches anti-money laundering laws your friend might be a genius coder, but he's not smarter than a entire government's legal team thats a fight you dont want.
Samourai is a centralized CoinJoin-style coordinator.
However, the fact that they were centralized was probably not even the main problem. The problem was that they worked and presented themselves like a company. And thus they were subject to AML laws. They took a fee for the coordination service. And there were also some quite big communication fails, they advertised themselves as a tool to evade sanctions.
Some in the "Mixer restrictions" thread have said that they could probably have avoided the legal trouble if they weren't so irresponsible in their communication.
There are CoinJoin coordinators who still are active, centralized ones (like Wasabi, who have however shifted to do AML checks to not expose them to the Samourai trouble) and decentralized ones (like Joinmarket).
And
Joinmarket does offer a wallet too, with the "mixer" integrated.
Perhaps if he wants to offer something new the coder could offer a wallet with various options for CoinJoining. If the coder takes no fee from the process, he is also not subject to AML laws. But as early as he gets any income (other than perhaps donations) he could be subject to AML laws.