I just got an H&M order last night, ordered a pair of shorts with no elastic in the waist, in an 8, 10 and 12 cos needed options
anyways I am a 10 on the bottom half and the 8 fit perfectly. So I think it very much depends on the item of clothing, fabric, etc.
She just needs to try on multiple sizes. At the end of the day clothes are made to fit us, the numbers on the labels just are a guide. If you’re not sure, order multiple sizes or go to the shop itself and head to the changing rooms with 3 or 4 different sizes.