Unlocked phones are nothing special. I unlocked my phone in like 2 minutes using free SGS unlock app from the Android market. SGS phones are probably the most popular Android phones out right now with over 7 million sold worldwide.
Problem is the guy is from Canada. From what I know, Canada uses 850/1900 also meaning he can get a AT&T phone and unlock it. But why? There's more choices out there with pre-unlocked phones.
The Aria is just like a Legend and the Captivate is just like a regular Galaxy S. There's no reason for him to buy an AT&T phone unlocked unless he really likes them.
That's why I suggested the Milestone (perhaps wait for Milestone 2?) or just get the HTC Legend. sorry I mentioned Hero but that's the old version of the Legend. If you're willing to shell out maybe the HTC Desire would be nice.