He really needs to get the wheels to fit his application. He needs to look at three things, as I can see it.
1) He needs to consider the bolt circle. The wheel has to physically bolt to the Mustang.
2) He needs to know the actual size of the wheel, to fit his existing tires, or to fit tires he plans on buying.
3) He needs to consider the offset, or backspacing. If he can find out what the specs on his current wheels are, that can make it easier. Otherwise, he needs to stick to looking through catalogs and looking at wheels listed for his car. Otherwise, it's purely guess work. It may work, and it may not. Again, finding the specs on his current wheels are important if he wants to buy wheels that aren't listed for his car.
The wrong wheels can mean many things. In general, by "wrong" I'm assumming you mean dimensionally. Assumming that the wheels properly fit the tires, that will just cause a clearance problem.
The other issue that most people forget about is load capacity. Wheels have a specific load capacity. If you put wheels on a 5000 pound vehicle that were only designed for a 3000 pound vehicle, you could have a catastophic failure at anytime. The wheel could virtually just blow apart with no warning.
In addition to the things Katshot mentioned, there are issues like wheel bearing life and alignment to deal with. It's really best to stick to the recommended wheels, or wheels listed to fit your particular application.
For example, all modern FWD Cadillacs have the same bolt pattern and same general offset. And they're all about the same weight, give or take 500 pounds. These wheels generally interchange with each other. If your friend knows about direct interchanges with the Mustangs (like Mark VIIIs maybe?), he can probably work with it that way, but forewarn him about the dangers that are mentioned here.