I'm sorry I can't help, because it sounds like a great and very original project, but I did find a web site with a protocol for measuring Vitamin C: here

If the student found a mentor that worked in a lab with the necessary equipment and chemicals, she could easily do a project studying different cooking techniques on Vitamin C content. She'd have to measure Vitamin C in solution, even after cooking, so it might be easier to use something that has a lot of juice, like a citrus fruit or a tomato. She'd have to standardize the amount of material she used to measure vitamin C (eg 10ml of juice). The different cooking techniques might be steaming, boiling, grilling on a BBQ, and baking or broiling in an oven. I imagine that the amount of vitamin C per fruit or vegetable varies a lot even if you use only one type (all lemons, for example), so she'd have to do several replicates to see a difference statistically. Maybe 3 or 5 or even 10. I know people usually don't cook citrus fruit, but they are juicy. If instead she chose to measure vitamin C in the cooking water, (a) there might not be enough vitamin C to measure with the above technique and (b) she'd have to use the same amount of cooking water and boil them for the same amount of time to prevent changes in vitamin C concentration just from differences in the amount of water collected to measure it.