But the target is 68k assembler, so he's gonna need an emulator, too. I was thinking Chubby Bunny plus some old IDE like, uh, well, I've forgotten. I dunno, maybe ThinkC? CodeWarrior? MPW?
I might try running Linux in a VM on a Mac, and find an old copy of the PalmOS (et.al.) 68k IDE (I think it was Eclipse based). The Codewarrior IDE might run under Windows in a VM on a Mac.
The closest thing you'll get to Assembler IDEs are glorified text editors. If you want a text editor that has file management look at Notepad++ on Windows or jEdit on *nix. Most good assemblers will have debuggers anyway that will happily spit out register values, next, skip, repeat etc.