

breast movement varies from annoyance to uncomfortable
As a teenager and young woman, not being able to find a good super supportive sports bra for my weird size was one of the reasons I stopped trying to exercise.
I still hold my boobs automatically when taking the stairs or when I run somewhere. Otherwise it hurts and feels like my breasts are being torn away from my body.
Kudos to your friend going through with a reduction to pursue her passion!
In my case, I have a very small band width, so I cannot shop in regular stores. (In my city, there is exactly one shop that has my size.) As a teen and young woman, I simply didn’t have the money to even consider a 50€ sports bra, let alone a 100€+ one. And since the selection is super limited, I didn’t even find one in my size that would - no matter the money - give enough support for comfort.
Funnily enough, regular wired bras with cups still ended up giving better support than any sports bra I could find. But they still only work so well. (And I, by far, wasn’t as passionate about sports as to get a reduction, or spend my limited bra money on a semi working, ugly sports bra.)