He already said he'd tried different accounts. And it simply won't work.
De-DRMing books is a permanent solution. It's a minor nuisance, I agree. But when you get done, you have complete control over the book you paid for. I don't see an ethical issue there, at all. As for whispersync features? Actually, yes you do. If you use Calibre, it puts the ASIN in and Amazon sees it correctly and saves bookmarks, etc., just fine.
Dual boot is almost never an answer. And virtualization is only an answer if his machine has the horsepower, which it sounds like it doesn't really.