It’s so bad that my fiancée has some bras that say she’s a B cup and others that says she’s a D cup. In order to go bra shopping, you have to actually try them on to find out if they fit.
If I had to try on underwear to see if they fit, I might not bother with underwear at all!
Have her go and get fitted. Many women don’t know what their band/cup size really is.
Also, IMO, women’s pant sizes are where the real absurdity in sizes is.
Not much help to know what cup size you are if the bra companies are only pretending to be standardized