The only problem with that is the loss of sound quality from not using a head unit. You may not notice it, but being in the business I notice it all the time. Compressed music sounds awful. Then you don't have tone controls, only the equalizer. I'm sure he already has a decent head unit though, and if so, there's not much need to upgrade it if he plans to add a WS-420.
Another good reason to have a head unit is to have separate remotes. My boat has one on the transom and I use it all the time. When we're on the beach, swimming off the back of the boat, before I go out on a set, etc....
I've still got the stock Kencrapwood in mine, but as soon as I can find the carbon fiBBer piece at the helm that's blank (without the remote cutout) I'll be upgrading mine to an Alpine.