If Her Maj' is still on the way to get some line, the best bet is to take the line drum off, then separate & clean the outer shell - the bit with the grommets in that the line has to go thru'. Put it in a plastic bag & get Her Maj' to take it into the shop with her.
Most rolls of line have a free end that you can check - just see if it will go thru the grommet, if it doesn't then it is too thick, try next size down. I've noticed that every brand seems to use a different colour system, yellow in one brand may be 1.6mm but could be 2.5mm in another.