First off, if your fish are in good health and you do a good water change prior to leaving they'll be just fine when you get back even without your friend. Second, throw the feeder blocks away. They do more to hurt your water quality than feed your fish. Third, buy one of those pill boxes from the drug store; the ones with multiple compartments. Put a regular feeding's worth of food in each compartment, just enough for the number of times your friend will come over. Tell him when he comes over just dump one compartment in the tank and that's it, only fill enough compartments to get him through the two weeks, nothing extra. Just use your flakes or food that keeps well. Don't give your friend any portion control at all. Don't mention the frozen food or the zucchini or anything like that. Finally, HIDE THE FOOD. Leave nothing where your friend can find it.
As much as I'm sure you'd love to know your fish are getting their varied diet during your trip it's not worth it. People who aren't familiar with fish do not get how small their food portions really need to be. Add on to it that since you'll be away and unable to do a water change you don't want them pigging out or leaving food in that can rot in the tank, like a slice of zucchini. So the answer is simple, don't give your friend the chance to over feed them. Give him premeasured portions and only those. Hide the rest of the food. If he questions how little food they are getting assure him its enough and maybe explain why they get this amount of food. Under no circumstances tell him where the food is.
I made this mistake with my tank at work one time. I was out sick for several days and called my secretary to feed my fish for me. Well I got better and two days later got into work. It looked like it had snowed tetramin flakes in the tank. Every horizontal surface was covered in food. I had to do an immediate vacuum and water change. She didn't do it maliciously, she just didn't know. Now unless I'm unexpectedly out for more than a week I just don't call. Unless I can prepare the portions ahead of time I don't ask someone else to feed my fish.
Oh, and if you're worried about variety there are a number of dry foods that store well you can use. You can go with a flake food, algae wafers, shrimp pellets, etc. so that the fish have variety even if its not fresh.