We once had a dog aggressive staffy who was, in every other respect, a lovely, lovely dog. This was years ago before behaviourists etc, and just had to muddle through. In the end we did stuff like walk her very early in the morning, or in the evening, or only take her to places where dogs were on leads... And it was fine. Honestly, fine. She was - as most staffys are - great with kids, and people. Just hated other dogs.
It was worth putting up with that, because in every other way she was lovely. So that can be the case.
I have a dog now who is not at all dog aggressive but a bit afraid of certain dogs and we don't let her off lead. But she gets walked for an hour or two every day and we have a large garden where the kids play ball with her, etc, so she gets plenty of exercise. I choose not to let her off because I don't want anything happening to her. She is a staffy cross and I know she would never start a fight but she might sure as hell finish it and then as the staffy, she'd get the blame.
I have also had an extremely dog friendly staffy who was no problem around any other dog, ever but he was gone for once or twice (usually by labradors!) And again, my fear was that he might have finished a fight another dog started and he'd be labelled aggressive due to his breed, even though he was totally good natured and calm around all other dogs (even aggressive ones). I got sick of people with off lead dogs having zero recall or control, and so eventually kept him on a lead, too as he wasn't the problem - but other people's off lead, aggressive dogs (often breeds with a very different stereotype to the staffy) frequently were.
It is something staffy owners live with, I'm afraid. I'd maybe offer to 'borrow' (foster) this dog for a set period of time, and be very clear that if unforeseen issues raise their heads, you might then pass him on to a rescue. That would give him a chance and get him away from a not ideal situation.
These folk shouldn't be breeding dogs, that's for sure.