Do you have a good vet? Perhaps s/he can give you some really good info you can pass on the the co-owner; coming from a local vet this may be better received. Just a thought.
Not only is frequent breeding a terrible idea because it just helps fill shelters with unwanted animals in the long run, (there are thousands & thousands of pure bred dogs waiting death in shelters every day) but as noted it's also very bad for the bitch. ...pregnancy and nursing take a huge toll on her, and 5-6 months isn't nearly enough time to get back to peak condition.
Hopefully you can talk this person out of it...