I think the best thign to do is relate it all back to finances. Either she cares about her dogs deeply, or she doesn't. If she doesn't, there aren't any appeals to ethics that will work because to her "it's just a dog."
Point out how many pitbulls are up for adoption in rescues and pounds. Geez, that's a lot more supply than demand? Think you'll be able to find paying homes for all the puppies?
Man, vet bills for pregnancies get expensive. What if there's complications? Can you cover all of the expenses if something goes wrong? You'd hate to lose out on your investment.
You should probably take the dog to obedience school. If people come to see your dogs and get a puppy, and they see a rambunxious or aggressive dog, geez, you might lose a sale!
If you can tell that she's going to breed no matter what you say, switch tactics and it's time to start lookign out for those poor pups. Research the laws in your area- in most places it's against the law to sell a dog to be used for fighting. The only way she can "make sure" (because you can't ever be sure) that they don't get used fo rthat purpose (and she doesn't get slapped with a fine, as I'm sure would be her concern) is to do home checks. Tell her it's easier to sell puppies with shots.
Also, be careful with those males- two unneutered males with no temperment testing and one unspayed female is a recipe for complete disaster!
Good luck!