"If a special outfit is designed for one teenager, it should be for all of them or none of them."
Why?
Only if the happy couple are obsessed with following mindless custom.
The OP hasn't indicated that that is the case.
I'm sure the people involved can come up with a compromise whether that means the girl is a bridesmaid in a slightly different outfit, or she has another rôle - as one or two posters have suggested.
More rude behaviour on public transport 😡

