I would assume that this person is in treatment with a phyiscal therapist, chiropractor or MD. I would suggest that you contact them to consult.
Personally, I would frame the question more general: what exercise movements are to be avoided? Once you know that you should be able to apply our knowledge to determine what exercises are appropriate. The reformer may not be the tool of choice initially.
If they are able to do neck exercises, have them what I call neck ups.
Same thing as a sit up.
Have them lie supine on a bench, feet firmly on the floor but with their shoulders on the edge, so their head (relaxed) is hanging. Have them connect their chin to chest.
Great for neck muscles that people really dont use.