All over a Sherlock DVD

[baskingseals]

Dd and her friend both love Sherlock. They watched a DVD belonging to her friend which dd now can't find. Friend is messaging dd saying that dd now owes her £18 to replace the DVD. Dd did not borrow the DVD, friend left it here after they both watched it.
Ainu to say I am not paying £18 to replace it? Also what should dd say to her friend? Girls are both 12.

£18!!! You can get the box set 1-3 for that 2nd hand!

I am leaning towards not replacing it if it can't be found, and I have already had a fairly good look in dd's mosh pit. But, I don't want her to use it as a little stick to beat dd with, as I think she might.
Thanks everyone, it is helpful to hear what you think.