Chow Chows and Sharpeis are totally different breeds, And to be honest a smooth coated Chow wouldn't look anything like a Sharpei.
Is there any way you could get a pic of the puppy?
I could look at a pic and tell you without a doubt what it is.
I'm going to say that the dog is either 100% Chow chow or that the puppy is actually a mutt and has both chow and sharpei in it.
The Shar-Pei is believed to have shared a common origin with the smooth-coated Chow-Chow. Chow Chows were bred as general-purpose working dogs for hunting, herding, pulling and protection of the home while Shar-peis were bred for fighting. They are two COMPLETELY different breeds, so you are correct.
Looks to me like your friend has bought a mutt, Shar pei cross with a chow, that's why the siblings were different.
You are correct in the fact that they are two different breeds. But there is a possibility that the mother mated with 2 different breeds. Dogs can mate with two different fathers and some puppies can look like each.