This can be contested, as the boyfriend still went "home" for a period of one or two weeks every month, and that most of his possessions were still in the "home" that he shared with his parents, and not that of his girlfriend's parents. And that nobody in their respective circles, themselves included, considered them to be anything more than just boyfriend/girlfriend. At the very least, there should be enough of a gray area to explain why the husband, then boyfriend, would not have considered themselves to be common-law at the time of his application. And neither did his wife, at the time of her application. Their relationship was most likely conjugal, but whether or not it was common-law is debatable - but not something that they would necessarily want to contest, either. But I'm not a lawyer. I don't even play one on TV.
Is this like with a child, in which if you didn't declare a child in your application, you are FOREVER barred from sponsoring that child? If so, I think they need the services of a real lawyer.