We tell folks here all the time that if a bridesmaid doesn't get her dress, she's taken herself out of the wedding party (assuming you discussed budgets ahead of time).
What is the protocol if said bridesmaid doesn't actually know that no dress = no participation? How do you say, "I love you; you're my best friend, but if you're not wearing the right outfit, I don't want you to stand near me." ? Is there really a way to come out of that smelling like roses? (Or at least not feeling like an ass hat?)
(In my particular situation it's actually FI's groomsmen, but I'm curious in a more general way since we haven't quite hit that point yet.)

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