I think it's a huge waste of money to get a cheap generic laptop. If you're going to spend $300-400 on a crappy laptop, you might as well get a more versatile tablet or 2-in-1. There are several 10" and 11" tablets that are more than adequate to do proper work on. That said, I'd even consider using an 8" one that has the ability to output to an external monitor and really get some work done. He obviously has some money to spend otherwise he'd never have purchased a Mac (they're overpriced), so stop suggesting the Parallels/BootCamp route. That's boring. Macs are boring. This is a good opportunity to get a really versatile device. Don't blow it on a dying breed (i.e. a crappy, generic, non-touch screen device).
These forums never fail to amaze me!
My buying decision Q/A looks like this:
1. Do you actually want an addition computer, or just to run a specific application which is not available for Mac? Bear in mind that a new computer will be more versatile (maybe) and exciting (if you like that kind of thing), but will be more expensive and mean that you have two machines to maintain, carry around (+ power supplies), transfer data between, etc.
- If just specific app: Bootcamp or Parallels mean that you don't need an extra machine and seem the best solution. You can spend the money saved on something more interesting.
- If new machine: there are plenty of Windows laptops available for under $500. The Asus recommended above looks like a good one. As I said, I'd suggest a trip to your local electronics shop to see what takes your fancy. Personally I also don't use touchscreens on laptops - I find them awkward and inconvenient, but the OP might have a different view. Anyway, even if you have a touchscreen, you don't have to use it so it's not a disadvantage.
A note on screen size - as I posted in another thread recently, even a slightly bigger screen increases the area quite significantly. For some applications, that's a big advantage. I'm not familiar with QuickBooks Premier, so can't specifically comment on that. And regarding tablet/laptop combination devices - I doubt you will get one running Windows 8 for $500. They are worth considering if you don't mind spending more and can live with a small laptop/big tablet device, and with Windows 8 as a tablet OS, and you don't already have a tablet. Personally I find a tablet too small for 'real work', and therefore have both a laptop and a tablet. But that's just my opinion. I recommend to try for yourself.
Good luck with whatever you decide!