Legally, the neighbours have no claim. There are plenty of moggies that look like Norwegian Forest Cats or Maine Coons, both breeds are "natural" breeds, they weren't created by cross-breeding, they're basically posh moggies*. Longhaired genes can be recessive so just because no male that your friend is aware of has that type of coat, doesn't mean that the neighbour's cat is the father.
Your friend could also add up all the costs of caring for the pregnant mother and kittens, as the mother needs to eat as much as possible and both mother and kittens need to eat kitten food. Then there's cat litter, heating, laundry etc. etc. So when that gets taken out of the equation, there is very rarely a "profit" for the greedy neighbours to share. Your friend could point out that even the pedigree breeders rarely make a profit, nevermind someone with moggy kittens.
The neighbours obviously think their boy is a "pedigree" making the kittens "cross-breeds" and therefore valuable. He's probably not a pedigree, as most breeders have a contractual demand that the cat be neutered. Your friend could ask to see the pedigree certificate. If the cat doesn't have one, he's not a pedigree, it's that simple. If it exists, it's almost certainly got the cat registered on the non-active register, which means those cats mustn't be used for breeding. Even if the cat is a pedigree, the kittens aren't valuable.
If the father of the kittens has a pedigree certificate, then your friend should make a careful note of the breeder's prefix. Say the cat's pedigree name was Starsmurf Furry Little Twerp. Starsmurf is the prefix name. Your friend can then try to find the breeder's contact details online or if that doesn't work, she can contact the Registering Body (eg GCCF, TICA, FIFe) and ask for help in contacting the breeder.
Let the breeder know that the father is
a) being allowed out without being in an enclosed, escape-proof garden (many breeders make that a condition of the sale)
and
b) un-neutered and has been allowed to breed, with the owners seeking to profit from this. This can mean that the owners have broken the contract they signed when they bought the cat. Technically, the breeder could even reclaim the cat. It's doubtful that they would but they might be "persuaded" to neuter the poor cat.
There's an old joke in cat breeding that sums up the amount of money to be made from it:
How do you make £5,000 by cat breeding?
Start with £10,000.
- Before anyone criticises me for being "nasty" about Maine Coons/Norwegian Forest Cats, I need to point out that there is a (rescue) Maine Coon snoozing at the end of the bed. I love him to bits, think he's a gorgeous cat and would have more Maine Coons/NFCs if it weren't for my cat not liking his own kind, due to growing up in a house where he had to fight for his food against 30+ cats.