did she try on the exact brand and model helmet she was going to order? if not, all helmets are different, arais fit different than shoeis and so on, and even different model shoei's fit different within the brand itself, so you could be a Medium arai quantum, and a Large in a arai rx7, or a Small in an icon

i never measure people's head with tape or go by hat sizes because it simply doesn't work, if someone wants a brand i don't carry i'll have them try on as many types that we have in stock until we figure out that they are generally a certain size, advise them on how a certain brand fits if i'm familiar and then order from there, giving them the disclaimer that it could not fit, in which case i will usually wave the restock fee if they want to order the next size up or down, if they leave me with the helmet and want a full refund and don't wanna order another one then i'll usually charge them whatever that company's restock fee and shipping is

so the not fitting thing would make sense to me unless she really did try on exactly what she was ordering, in which case ordering it wouldn't make sense unless they just didn't have the color she wanted or something, but otherwise you have to be cautious since no two helmets are the same