I’m 5’4" and a 10DD (I think that’s a 32-34DD) - and avoid standard t-shirts like the plague. If I do wear them I can get away with a small (I’m narrow through the back, shoulders and small waist), but they tend to hang directly down from the twins giving a boxy shape I hate. I would only recommend you buy the shirt if it’s a little shaped, in which case I’d go with the medium given her size).
Better yet, buy her shoes - regardless of the chest size, shoe size is pretty standard, and most girls love shoes! (Me anyway…)
Absolutely. Anyone who is built that disproportionately should be wearing tailored clothes. Otherwise, it looks like you threw on a tent. That look is never cool.
Well I’m 5’4" with 34 D, and I can usually do fine with a medium shirt. However, I have not heard of this thing called a “standard medium”. I’ve only ever been frustrated because with women’s clothing there is very rarely ever standard sizing, and it very much depends on the brand.