Just eating healthy foods will make a HUGE change in the majority of people you will train. Complex carbs, good fats, real meat (not the processed and preserved bs), ect will make them feel better, shed fat, and aid in gaining muscle.
Theres some "quick and easy" results you will get credit for. The diet forum has a TON of info. As for special supps, there really are none. They are just that, supplements. They don't do much if the person doesn't take care of themself or eat right. But some that are good are:
Fish Oils (omega 3 fatty acids)
Creatine (if strength and size is a goal)
A post workout protein shake is nice if they can't eat a solid meal afterwards
Amino acids
Multi-vitamins (most people don't get all their vits covered with whole food)
Calcium (especially for post-menapausal women)
Stay away from any "gene-altering", "incredible muscle building", "mister olympia endoresed" stuff. Its all hype with little or no evidence behind it.