The significance of a match with another surname depends on at what level (how many markers) and how close the match is.If the match is at 12 markers and a very common haplogroup (as R1b1a2), then many people have well over 1000 matches with different surnames. Many of those matches will fall away at higher levels of testing.
Even at 37 or 67 markers, there may still be matches that go back to before surnames came into existence. With a match with a Christensen, it could also be your nephew's ancestry and his match are from a country that practiced patronymics, where the surname changed each generation (as [first name] Christensen would be [first name] "son of Christen."Christen in turn might be known as Christen Hanson [Christen son of Hans], who might be known as Hans Williamson [Hans, son of William, etc.] Some countries still practiced this up to around 1900.
You also have to consider that just as today, ancestors adopted orphans and gave them their surname, and children were born outside of marriage and took the mother's surname, etc.Say this happened in 1500. While matches after then would have that that surname, matches from before that time would be with the biological father of a different surname.
The significance of your nephew's matches really depends on how close the matches are, and at what level of testing.