Here is the deal, he is either going to keep the fish or he isn't. While the most responsible thing is likely to take the fish back, he decided he is going to keep them, who knows, maybe the LFS
wont take them back (the ones around me have reservations regarding these types of things).
So, if he is keeping the fish regardless he really only has 3 choices, from how I see it, from most "harmful" to least.
1) Cycle the tank with the fish in it and wait
2) Cycle the tank and do a plethora of water changes
3) Add marine bio spira or a similar product
Of course doing it the natural way is the best, but other than returning the fish, he doesn't really have alot of options. Bio spira or a similar product is the only way he is going to keep the fish and maintain minimal stress.