If you know that a potential wedding guest is going to cause issues by badmouthing the bride at the wedding, then it makes perfect sense to drop her from the guest list. This is a "based on past behavior" issue, not a "wrongful exclusion" issue. Also, the LW isn't one of the people getting married, so I'm not sure why they get a say in the matter at all? The whole letter screams None Of Your Business to me.