I dont think the cooling off period applies if she entered their premises and made the 'purchase'... If a salesman enters your premises to sell to you and you agree - then you have 14 days to change your mind.
She may end up losing her money - as its really not their fault that she has changed her mind - even if they are "car salesmen"... She really should have thought twice - maybe this will make her think a bit more next time! A car is a major purchase and needs careful thought and research!
Try looking on some consumer rights websites...
http://www.consumerdirect.gov.uk/