The way I read 61.63 he needs training to proficiency, a CFI signoff that he's provided training and finds him competent and a checkride. No written required since he'll already have a commercial license with rotorcraft/helicopter category/class.
Right about the written, but not on "train to proficiency". Since this is an additional category (Airplane), not just an additional class (Single Engine Land), he must meet all the experience requirements in 61.109(a) to add an ASEL at the PP level to his CP-RH ticket -- see 61.63(b). So, assuming no prior ASEL time, he'll have to do the following:
(1) 3 hours of cross-country flight training in a single-engine airplane;
(2) 3 hours of night flight training in a single-engine airplane that includes--
(i) One cross-country flight of over 100 nautical miles total distance; and
(ii) 10 takeoffs and 10 landings to a full stop (with each landing involving a flight in the traffic pattern) at an airport.
(3) 3 hours of flight training in a single-engine airplane on the control and maneuvering of an airplane solely by reference to instruments, including straight and level flight, constant airspeed climbs and descents, turns to a heading, recovery from unusual flight attitudes, radio communications, and the use of navigation systems/facilities and radar services appropriate to instrument flight;
(4) 3 hours of flight training with an authorized instructor in a single-engine airplane in preparation for the practical test, which must have been performed within the preceding 2 calendar months from the month of the test; and
(5) 10 hours of solo flight time in a single-engine airplane, consisting of at least--
(i) 5 hours of solo cross-country time;
(ii) One solo cross country flight of 150 nautical miles total distance, with full-stop landings at three points, and one segment of the flight consisting of a straight-line distance of more than 50 nautical miles between the takeoff and landing locations; and (iii) Three takeoffs and three landings to a full stop (with each landing involving a flight in the traffic pattern) at an airport with an operating control tower.
Based on some folks I know who've done this, it will probably take about 15-20 hours of flight training plus 10 hours of solo to be ready for the practical test.