Depending where you and your friend live, you will likely have to expand that $500 budget a bit.
Avoid monitors with Trinitron and Diamondtron tubes in them if the two lines are an issue.
NEC, Viewsonic and KDS all have non-aperture monitors that are very good. KDS will likely be the least expensive. NEC periodically sells "refurbished" monitors that can be better than brand new and a darn sight cheaper.
As I always advise, your friend should closely examine the monitor in a store, and resolve his purchase to the serial number. No matter the make of the monitor, there are often noticeable variations in adjustment from one to the next, so he needs to be picky...
Good luck...
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