There are too many ways to slice the pie on this. One of the common ways I have heard is having a nutritionist create a diet plan for people with special conditions (low fat, high protein, heart healthy, diabetic) and include that as part of a client’s training package.
The other way is to collaborate on a video, book, or other form of media.
I personally would love to have a nutritionist speak on my fitness video on simple things a client can do to control their diet.
I can’t provide any recommendation on the do’s/don’t because I have not worked with a nutrition coach in a partnership format.
Best of luck.