Give her a variety of 'back sizes' They go on the Loosest hook. To check for back she puts them on at the appropriate height (strap at the underside of the breast, band level), but this is easiest with the cups in the back. 2-3 of her fingers should fit under the strap, but not more. It holds most of the weight off the shoulders once she has support needs.
With the correct back size, select a variety of cup sizes (she is likely to start with AAA, AA, or A), and fit with the cup in the front. Different brands will have different spacing and shape of the cup, but the volume is "supposed" to be similar.
A 30B is a similar size to a 28C or a 32A.
When putting on the bra she should lean forward (and when relevant 'scoop' the breast tissue into the cup). Initially this won't be a problem.
Most women (and most high street shops) have no idea about fit of bras, even those which claim to offer a fitting. The end result should be a tight band which just allows her to place 2-3 fingers under it on her ribs, with a cup without excess volume, but without it cutting the upper edge of the breast into 2 or 3 volumes.
Different styles, or manufacturers will give different fits from the same nominal sizes, so she has to try each on and select only those which fit - comfortably and without obvious gaping or bulging.
High street stores may have limited ranges. Online stores such as "Her Room" may give a wider range of styles.
Initially, she may not need much beyond a 'training bra' or 'sports bra' in a suitable band - this is more what she should look for in adult sizes once her breasts develop to the point she needs more.