Lack of food is not really an issue if the person in question is otherwise healthy. However, lack of water certainly is. And depending on sequence of duty, lack of coffee or other stuff that wakes you up as well. Shouldn't happen, but i believe caffeine is mentioned in every fatigue risk training i have ever witnessed. Not as a long term solution, but to help in a pinch.
That mentioned, of course the number of possible issues that might cause someone to faint are numerous. I hope the FO in question recovered quickly and has no lasting problems over that.