I would be working through some suitable responses with them, so that come back at their 'friends' with some light hearted retorts that puncture the nonsense of being big chested equals 'popularity'.
Dd1 is 14. Her friends are all different shapes and sizes. She spends a large proportion of the week in dance wear. At costume time, the dance school have a hell of a time with ordering sizes, as at this point girls' shapes change practically overnight. It's not an issue for any of them and I have never even heard it discussed. Some girls are extremely petite, completely flat and boyish figures, others are much heavier with very obvious hips, boobs that need a great deal of support not to be distracting when they are dancing...
Variation is pretty much the name of the game in body shapes. They need some support to understand this, not validation that the expectation towards a culturally acceptable 'sexy' a la freaking la Senza giant wonder bra window display is normal and should be encouraged.
I wouldn't be buying new padded bras for teens, unless their concerns were more towards nipple coverage (ie a t shirt bra to give a smoother outline for modesty), rather than faking big tits.
I'd be concerned about what the message 'yes, yes, be like your friends' was going to do to them in the longer term. And I'd also not want them to turn out like these particular 'friends' to be honest.
Do they do anything with their bodies that helps see them in any other light other than pre-sexual? Any sports? And hobbies that require strength, agility? Any thing else that might encourage them to move away from the purely aesthetic and realise what a load of old crap it is?