Was she in full-time education? If so, she wouldn't have to pay, although your son would have to pay. Not sure if it is applicable here, just thought I'd let you know.
Also here, Citizen's Advice Bureau,
http://www.adviceguide.org.uk/nm/index/family_parent/housing/young_people_and_housing.htm says under 18's do not pay council tax..
http://www.adviceguide.org.uk/index/life/tax/council_tax.htm this confirms it. It appears your son is solely liable.
If either of the above, then she is not liable to pay, but your son may be able to get a 25% discount as a sole occupier. He isn't actually, but he is the sole occupier liable to pay the council tax. We had this situation at college where one working person lived with two students and they got 25% discount on sole occupier basis (as students don't have to pay, although do need exemption certificate from place of education).
Once 18 though, she would have to pay, so for those six weeks, your son would not get a discount and the full amount due on the property would have to be paid.
Hope this explains it.