Can dogs get diabetes from eating bread?

My boyfriends mom has three small dogs. The youngest of the three is a Cairn Terrier mix. His mother feeds the dogs people food...she says she only feeds them bread and cheese, but she feeds them a lot...and i mean a lot of it. I didn't want to say anything to hurt her feelings by telling her not to feed them bread and cheese, because it's not my place to tell her what to do with her dogs. But now, the youngest has soo many problems. Bodily tumors, thyroid infection, joint problems and fungus in the feet are just a few to name. But today, her vet told her that the dog now has diabetes. Is it definately from the bread? What should I do to tell her without hurting her feelings?