I honestly wouldn't just 'ignore' the matter, as this could seriously damage your daughters future, say she wanted to go into law and there was a judgment against her or she found out that her credit was tarnished when applying for a loan.
Contact the police and ask their opinion on the matter, i wouldn't dial '999' but get a copy of the local police stations number or better still, go in and have a chat.
You could of course, contact a solicitor and ask for their opinion, but to do that would be counter productive as you don't want to pay out any money.
If.. in the case you would like to forget this whole matter, i would strongly suggest paying what they are asking for, but get a copy of exactly what the damages are for, if the stolen items are only for fifteen, what does the rest cover etc etc.
and maybe keep your daughter away from her friend. (but i highly doubt one was stealing while the other was oblivious)