My answer to your title question is: it depends on the circumstances. Judging this couple based on a statement she heard in the RECENT past is completely understandable. Even if the statement was not like them at all, it was still said. So at some point they were disrespectful to her, when they thought she wouldn't know. I wouldn't want to have anything at all to do with them if I were her. That includes being friends, metamours, or anything. Now, if she had heard secondhand stories of them saying or doing something like suggesting one of them fuck someone else to get rid of that someone else's significant other for the time being, I wouldn't say that it's okay to base a judgement on those actions.
Basically unverifiable past = not fair. Verifiable and personal = fair.
Now, I also agree with nycindie that your focus really seems to be on sex, and maybe you should reevaluate what you're really looking for and possibly adjust to a more swinger mindset vs poly.