european outlets in an american building
The National Electrical Code used here in the US requires that equipment be listed, marked, or approved for the use. The requirement varies with the particular part of the electrical installation. Receptacles should be listed. However, that doesn't mean they have to be UL listed. The Canadians have the CSA, the Europeans have the CE, and I am sure in Asia they have something else that I can't remember. See here.
So, the issue with an inspector could be that the receptacle(s) you install is non-standard and they would probably disallow it for satisfying any requirements of the code. At least that is what I would do. But if you want to add non-standard receptacles in a house, there is nothing in the code to prevent it. For instance, if I wanted to wire my house with DC receptacles in places where I want to use DC equipment directly from a DC system. Or I want a 240 volt receptacle next to a window for a large plug-in AC unit.
One of the things the code requires is equipment with different voltages must have receptacles that are different enough so they are not interchangeable. So 120 volt equipment cannot possibly be plugged in to a 240 volt outlet.
So, it appears that you could satisfy the code requirements by adding an extra receptacle (not by replacing an existing one) with a euro-style plug. Also you need to make sure their equipment is rated for 240 volts or you will need a transformer to step it down to 230 or 220. Then you wire it like you would the aforementioned 240 volt receptacle. (Standard NM cable in the US contains wire rated for 600 volts so the voltage rating of the wire is not a problem.)
Good luck!