Yes your friend would taste the difference. Why do you doubt him?
Fat is not necessarily a flavoring (although most fats do impart an important flavor to food,) but they’re very important for carrying the flavor of certain spices (such as paprika, which is not water-soluble) or for keeping meats moist during roasting and stewing. Try to make a homemade sausage with lean pork and you’ll get a ghastly hump of meat with no flavor and the most god-awful texture. Trust me on that one. I think America has gotten way too crazy on this lean meat trend. Better to eat less meat with more flavor and fat then to subsist on poorly prepared lean meat. (Don’t get me wrong; lean meat has its uses, but more often than not, it’s badly prepared and dried-out.)
Salt is also important as an agent which melds flavors together. It is not simply there to make something taste salty. It also has the ability to enhance and bring out flavors naturally occuring in food.
The problem with most low-fat, low-salt food is that it tries to emulate high-fat, high-salt cuisine. I’m sorry, but no matter how hard you try, you’re not gonna create a respectable low-fat cheese and pepperoni pizza. So why try? Same problem with vegetarian food which tries to emulate meat. What the hell’s the point? If you like meat so much, then eat meat dammit. Good vegetarian food is wonderful, and never pretends to be a “veggie burger” or “wheat sausage” or some other lifeless, unimaginative abomination.
And don’t even get me started on rice cakes.
Point is, you’ll be hard-pressed to fool your friend. The closest you may be able to come is barbecue grilled chicken breast or something like that. Otherwise, forget it.