I had my son come over every other day and feed them while we were on vacation earlier this year. I wasn't worried though as he had tanks before I did and is also a member of this forum. Even at that, I still had everything premeasured in cups so all he had to do was pour it in. Even though he has tanks of his own, he wouldn't have know how much I usually fed them. The main thing is that since he has fish too, he would have known what to do if there had been any type problem.
The only way I would trust a non-fishkeeping friend or family member to do it is by having the food measured out so all they had to do was pour it in. I definitely wouldn't leave it out to add at their discretion. As a matter of fact, the food would be put away where they couldn't find it to overfeed if they wanted.