Do not allow him to take a puppy out to the car to meet his others - as people have already said the germs etc could harm the pup and the rest of the litter and Mum may be funny with the puppy once back in with the litter - I have known Mums to kill their own if they are worried etc. Chance mating??? an accident??? and if this is the case then the owner of the stud is NOT entilted to a puppy back hes trying to pull the wool over your eyes with that one. Stand firm on this with the puppy meeting his other dogs - his other dogs could try to bite the puppy - it only takes a second for an accident like that to happen.....
I too shall obstain from commenting on cross breeding - I think my views on the matter are well known.