Well, I suppose you could make it work eventually, but no it will not work in a Mustang without extensive custom work.
If he was after IRS he'd be better of starting with a late model Cobra IRS.
If he's thinking that the rear end is a solid axle then you can tell him the 89-97 Bird/Cougars have IRS, and what you have is essentially just the very middle of the 8.8 axle. I expect that what he's looking for is an 8.8 out of an 1987-88 Turbo Coupe.