Societal cultures differ, but for most people in the Western world, the United Kingdom as much as the United States, a married couple lives together in the same house and sleeps in the same bed. If that's not the case, there's something wrong, i.e., the couple legally separated in anticipation of a divorce, or the marriage is one of convenience only, entered into to obtain immigration benefits, etc.
None of this is good. I have no idea why your niece and her husband decided to get married if they live many thousands of miles apart, but if she ever tries to get an immigrant visa for him, they will have to explain this to the consular officer in London.
Since they are married, he is indeed risking to be denied entry at some point. There's no guarantee if this will happen, or when, but there's also no right for a foreigner to enter the United States either. Perhaps your niece should visit her husband come next vacation time.
An immigrant from Europe, I live on the American Riviera and work as an attorney in Santa Barbara, California.