ToHo, I don't know where your customer is from. But in my shower, if I put the water on while I'm getting undressed. The water is hot enough to warm me. Maybe you should get the plumber to lable the HOT & COLD knobs.
I hope you can enjoy the humor.
But, sorry I personnaly have not been asked of such a heater. The only heaters I've ever installed aside from the central unit was your basic H/V/L types in bathroms. But I'm sure there are tons of different units that I have not seen yet.
Good luck in your search!!!