I only went to one place in HKG (co-workers took us there, mostly locals, so assume it was decent), and while it was really good, would not say it was any better than our local go-to here in Los Angeles - Empress Pavilion downtown. So either they are both average, or both really good (my guess is the latter - never disappointed with a Sunday meal at EP). It is a huge place with lots of carts - so perfect for your friend - definitely the place for his research in Los Angeles.

A few months ago I ate at Wing Lei at the Wynn in Las Vegas. They had several dim sum selections. Ordered a few and they were all amazing - definitely the best dim sum I have had. Of course you would hope so from a place with a Michelin Star. Not sure if they do cart service, though.