I'm waaay older than her, have literally flown hundreds of times and have never been aware of a dress code
Oh youd be surprised 
But thats the point im making....if there is a dress code then tell people
And thats not a bra...in different shops its a bralet and ive also seen it as a vest top and a crop top...dd has a bralet and there is much more material on hers
So even if they said ...dont rock up in your bra, she would still wear it as its not a bra