I'll wager an opinion here.
If this is truly dietary counseling on a specific, established illness (i.e., obesity, DM, etc), then you would report the appropriate E/M level (99201-99215). If the provider spends greater than 50% of the visit counseling the patient, then the E/M level may be based on time (making sure the CMS requirements are met).
As for an ICD-9-CM code, again if this is truly dietary counseling, report code V65.3 as the primary. For the secondary code, I would recommend you report the disease or condition that the counseling is being provided for (i.e., obesity, DM, etc). A third code for the patient's BMI could also be added if it is documented.
Hope this helps and again this is my opinion.