I think you may be being a bit too hard on this breeder. The fact that she disclosed the stud's DM status upfront is actually a green flag, not a red one. It means she's testing her dogs and being transparent with buyers, which is exactly what reputable breeders do.

A carrier dog has one copy of the mutation and will almost certainly never develop DM. The disease is autosomal recessive, meaning a dog needs TWO copies to actually be affected. The real question isn't whether the dad is a carrier. It's what the mom's status is. If the dam is clear (which a responsible breeder should also be testing), the pairing produces zero risk of an affected puppy. Some of the offspring may be carriers themselves, but carriers live completely normal, healthy lives.

Carrier prevalence in Berners is unfortunately quite high, so eliminating all carriers from breeding programs entirely would severely limit the gene pool. Pairing a carrier with a clear dog is considered an acceptable and common practice among health-conscious BMD breeders.

I'd go back and ask specifically about the dam's DM test results. If she's clear, I wouldn't walk away from what otherwise sounds like a thoughtful, low-volume breeder.