Without evidence to the contrary, the individual's first marriage should be presumed valid, thus making this one a farce. As such, you can't really show up because attendance at such an event is to tacitly approve of it and "celebrate" it, and in a sense witness it. That's why weddings are public events, so they can be publicly witnessed. If a wedding were in a Catholic setting and you knew of reasons why the marriage would not be valid, you'd be required to come forward with the information.