This is an area that I am very familar with. The issue with me and Macs early on was it's inability to run AutoCAD. AutoDesk claimed they were once working on a Mac version but I never saw this come to light and since I didn't have a Mac at the time, I did not care. For this reason, I must keep a PC in my home at all times.
There are MANY programs out there that will allow you to create floorplans and elevations (design homes) on a Windows machine. However, on a Mac this is going to be hard.
The obvious favorite is AutoCAD a super pricey program, but it is the best and it's industry standard. I own AutoCAD 14 and 2000 outright, but again, this doesn't help you with your friend's Mac issue.
Chief Architect is the next favorite. Very nice renderings can come from this program. But alas, not a Mac program.
Abracadata Design Your Own Home Suite is a "design your own home suite" that will do what your friend needs it to do. If he is an architect or something along those lines and needs to ensure whatever he does can be sent to a machine with AutoCAD or something like that, then he'll want to really research this product. It seems to only run on a mac. AWESOME! That means it will not have issues that you sometimes get when you have a windows specific program ported.
Required Operating System: Mac OS 7.5, Mac OS 8.1, Mac OS 8.5, Mac OS 8.6, Mac OS 9, Mac OS 9.1, Mac OS 9.2
You may also want to check this out, don't know if it runs on a mac.
http://www.punchsoftware.com/products/as4000.htm
Well, I hope that helps. Honestly, if your dad needs this for the reasons I think he does, the Mac may not be his best option. It'll need much more research than I can do right now...FedEx just dropped off my pbook so I am outtie.