Your friend, who lacks an instrument rating, would be the sole manipulator of the controls of an aircraft conducting an operation (IFR flight). Since he is rated for the aircraft (not necessarily for IFR flight), he may log the time as the sole manipulator of the controls.
This is a frequent question, and there are numerous resources addressing it.
A plain language reading of 14CFR61.51(e)(1)(i), which states: "(e) Logging pilot-in-command flight time.
(1) A sport, recreational, private, commercial, or airline transport pilot may log pilot in command flight time for flights-
(i) When the pilot is the sole manipulator of the controls of an aircraft for which the pilot is rated, or has sport pilot privileges for that category and class of aircraft, if the aircraft class rating is appropriate;"
There are no other limitations which require the sole manipulator to have an instrument rating.
Furthermore, the pilot, acting as PIC, who is responsible for the safe conduct of a flight in IMC, and is rated in the aircraft, with a valid medical, and is current, is able to log time acting as PIC, whether or not they are the sole manipulator of the controls.
Therefore, the sole manipulator may log as PIC time they are the sole manipulator, and the pilot acting as PIC may also log time they are acting as PIC.
This topic frequently comes up and DPEs and Flight Instructors are fielding this question frequently. Not everyone gets it right all the time. Personally, I have had scores of instrument students take flight tests with DPEs and FAA Ops Inspectors, and I believe everyone of them has logged time in the manner the OP asks. I urge pilots to add details in their log book remarks, describing the situation and conditions, and while it might not be required to also include the name and cert # of the other pilot in shared duty flights.
Some clubs, and FBOs may have issues with who is responsible for flights, and you might consider memorializing your agreement prior to your flight. Also note that there is no reason why the acting PIC could not switch during a flight one or more times.
There are quite a few assumptions in this answer, such as there is no type certificate requiring multiple crew members, etc.
BTW, if the manipulator in this example, stays under the hood, they could continue to log as simulated instrument flight, even though the conditions are actual. Otherwise they may log instrument time, and since they are not rated, in this case, I would very strongly urge that the remarks section record the name and cert of the pilot acting as PIC, who would be required to have current instrument rating (and who could not log instrument time for that portion of the flight, or log any approaches, as they did not operate the aircraft for the approach).