I am making the assumption that this Soldier is fasting for religious purposes. Based on my understanding of Army Regulation AR 600-20 paragraph 5-6, commanders will make reasonable accommodations for a Soldier’s religious beliefs. If the fast will interfere with the mission or potentially cause harm to the Soldier in the current operational environment then perhaps it would not be appropriate for the Soldier to fast. The Commander should make the call. If there is no impact on the Soldier or mission there is little reason to worry.
When working through this problem it is important to look at it from another perspective: Sometimes Soldiers go on diets or even stop eating at as result of trying to lose weight or they just may not like the food that is available to them. Some individuals fast frequently for health related reasons. Most individuals do not make it a point to inform members of the chain of command that they are fasting or dieting as such these Soldiers are not usually counseled.
It is important that Soldiers understand if their behavior endangers themselves, others, or the mission they can be held responsible for their actions under the UCMJ. It is not normally applied to situations like this but it can be if required.
I would encourage you to discuss this issue with the local Chaplain, JAG, and your Chain of Command before proceeding to ensure you handle this issue appropriately. Based on the information you have provided me regarding this situation I would encourage you to recommend a course of action. It is my personal opinion that the commander will have to make the final call on this issue especially if it deals with a religious belief.
A counseling for fasting might begin with something like this:
___{INSERT RANK/NAME}, on ____{INSERT DATE} you brought it to my attention that you intend to fast for the next ___{TIME FRAME} Days. While I understand the benefits of fasting I must also caution you against the dangers of fasting. Fasting can have adverse effects on your ability to perform and could also endanger those around you. I am referring this matter to the Chain of Command for resolution with the recommendation that you obtain a medical opinion regarding your desire to fast. I must also inform you that should you choose to fast and should your actions harm yourself, endanger others, or have a negative impact on mission requirements you could be held accountable for your actions under the UCMJ.
Draft your intended counseling for review/approval by the chain of command before you show it to the Soldier.
Read The Mentor: Everything you need to know about leadership and counseling for more information about Counseling, Leadership, Corrective Training, and Separations in the Army.