Her mere existence signifies a deep betrayal in Angel's friendship with the bride. After all, she had believed she knew her well. They were so close, intimate even, sharing clothes, family secrets, sleeping in the same bed. Angel realizes when this girl shows up at the houseboat that you never really know another person at all. Whole dimensions of a best friend's character can be fully concealed, and leave you exposed at the wrong time.
Marriage is frequently the biggest challenge they've ever encountered.
Sometimes, it's their weakness. Admitting defeat is just not anything they can accept without altering their self-image. Making others accommodate them is a source of pride. We all have recovered from mini-traumas suffered earlier in life. Childhood and young adult events are a ticking time bomb.