Maryland's Driver Handbook states that any nonresident of Maryland may drive on the MD roadways provided that the nonresident is in possession of a valid license to drive issued by the state or country in which they live. Drivers with a valid foreign driver's license may drive the same type or class of vehicle in MD that the person is allowed to drive in their home country, as long as they meet the Maryland age requirements.

It appears that your friend will not have trouble driving in Maryland with his British or UK license. If he only had an international permit that would not be enough to drive with but a valid license from his home country of Britain he should be able to drive in Maryland and other states.

For more information regarding driving in Maryland check with the MD Motor Vehicle Administration.