When I am giving lessons to my beginner students, I always include horse care, health, etc with them. I don't believe in tacking up or grooming my clients horse for them (unless they are too young) or anything like that. You could always try to include some information on conditioning/nutrition/body scoring with the client. However she may not be open to your advice.
If that's the case, I agree with your husband in that it's none of your business. It sucks for sure, but I wouldn't turn away a client because they were a little thick-skulled. Maybe if you take her on and she begins to trust you as her instructor, she may open up to the idea of changing her horses diet.