No, it's not true a stud wouldn't mate a bitch if she'd been successfully inseminated, but a regularly used stud dog will know if a bitch is ready/receptive, the two don't always go hand in hand, ie a bitch isn't always receptive at the optimum time to produce pups. My bitch had 8 pups although we didn't achieve a tie, I would honestly suggest if your friends dog isn't KC registered to tell her to look at having the mismate jab for her bitch, the drug is called Alizin, and needs to be adminstered twice to be effective, I think 48 hours apart.
The market is flooded with pups, if your friend has done no health tests, and the pups are not KC registered, they will just add to the overwhelming problem of the number of pups placed in homes that then disappear into the ether and somehow end up in rescue. That might not be their intention, but it's difficult to prevent when people aren't as clued up as they perhaps *should* be about dog breeding.
I don't mean any of that to sound negative or personal, it is just my honest opinion/advice, things can and do go wrong with bitches during pregnancy and unless you've done an awul lot of research and have a good reason to breed, I would say don't go there
