HA! Change the avatar plz, I see that pic and I'm thinking Mayne and I'm ready to rage.
OP, is the notice in this format?
http://www.dslreports.com/forum/r24527937-Digital-Copyright-Act
If it is, that should list the client used. Since she lives in your house and used the internet you pay for, go through her hardware and hunt down evidence, she probably uninstalled the client after you talked to her. Like the other poster suggested, check her prime too.
For me, the biggest offense here is not the pirating, if she indeed torrented the file, it's lying about it. Innocent until proven guilty is the right approach, especially since you can search her stuff and have the moral high ground to go through her stuff.