I'm not an expert but I have Polk in-wall surround speakers (model???) and I have always felt they sounded pretty good. Also, very easy to install. Make sure you don't have an upstairs heat duct (I did) before you cut the hole (large drywall patch required).
Most of the lower-priced in-walls are glorified car speakers. At that price range, unless he can use something that will mount on the wall (e.g. if he wants Polks you may have him look at the RT15i which comes in white and is in the same general range and 8.5 inches deep), the Polks will work, and as the prior post suggested, you may wish to be careful as to what is behind it and use a stud finder too.
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