
When mum takes the weak pup away does she only hold it in her mouth, or does she try and snuggle with it?
Does the pup make any sort of noises? like grunting or screams alot, and is his breathing like the other pups?
Does his breathing sound 'crackerly'?
Get your friend to check inside the pups mouth, if she is not sure what she is looking at, then compare it to the fitter pups, are there any abnormal holes, in the roof of the mouth? They can be very very tiny holes so can sometimes be hard to see.
Your friend is doing all the right things in keep an eye on him, but I would take him to the vet to have him checked over to put her mind at rest.