Ok, it sounds to me as if she's in totally the wrong size! I would guess she needs to go down in the back and up in the cup. How old is she? If she's a young teen then I would be very surprised if she measures 34 under her bust. My 13 year old size 6/8 niece who has no boobs to speak of, wears a 28DD! This is what she needs to do:
OK, here goes. Measure under her bust (hold the tape tight) to get her band measurement. Do not add on any inches. Then measure across the fullest part of her bust. Each 1" of difference between the two measurements equals a cup size.
So, if, say she measures 30" underneath and 35" across? That would give her a starting point of 30DD.
The best way I've found to test the band size is to put the bra (in the example above a 30 back) on back to front. If she can breathe but if feels tight and sheu can only fit two fingers underneath the wire at the front, then that's the correct band size. If it's so tight it's unbearable then she need's to go up a band size.
Once she's got the back sorted, it's time to get the cup right. The cup will differ much more between brands and styles than the back (IMHO). She needs to turn the bra round the correct way, lean forward and using her hands, scoop all her breast tissue into the cups (making sure she get all the flesh from round her back and under her arms in). Then she should put the shoulder stup and adjust as necessary (not too tight as it's the back that should be doing most of the supporting work). If for example, she's started with a 30DD, she will probably find that after scooping, she has some bulging over the cup and quadraboob going on! This means yshe need to go up one or more cup sizes. Keep going up through the cup sizes until all her breast tissue is firmly in place.
A D+ is not big! I do not know anyone (young or old) in my friends/family circle who is smaller than a DD.
Most people's boobs are different sizes. Best to fit the larger one and then use padding/chicken fillets to pad out the smaller cup if needed. Ewa Michalak makes bras with removable pads to help with exactly this issue.