Lost or stolen puppies?

My friend has two Bull Mastiff dogs, and the girl one had 4 lovely puppies about 9/10 weeks ago. One died shortly afterwards as it was the dog's first litter. My friend takes exceptional care of them and he would let them out in the garden when he was there. His lane down to the house is half a mile long so there is no chance of the puppies reaching the road before he would realise. Recently they would follow the mother dog around the garden. My friend went outside the other day and saw the mother dog come down the lane, without the pups who had been following her. They have not come back. He has looked for them but they couldn't be found. One of the neighbours took his girl dog before but my friend managed to get her back. They were very valuable pups, my friend thinks they were stolen as they wouldn't have left the mother dog. He is worried about them because they are so young. Is it possible to report them stolen, if he isn't sure? Where might he go from here?