MC? Sure'nuff! For 1989 and 1990, rear discs were only offered with ABS it seems from the parts lists, and if so, as you're well aware, there's a BOATLOAD of difference.
However, the balance won't be that bad between them.
It'd still be an upgrade, IMO, to go disc rear even if he keeps the old drum brake MC/prop valves.
I converted my 95 LX from rear drums to discs. If your car doesn't have ABS, there's a 96-97 factory proportioning valve for 4 wheel discs but no ABS. Can't remember the part #, but it's what I used in the conversion.
89-92 discs use the same caliper as the Fox Mustang, so there are a ton of pad choices. I prefer the 93-97 calipers, though, as they're better for a Cobra rear disc swap should you choose later on.