Tor hides your traces from outside, but really, it dosen't do anything for what the system stores. The tor browser bundle shouldn't store anything, but the fact that its there might be a clue that its been used - especially since it needs an install.
I'd probably just make a fresh copy of the windows XP VM she's using (by exporting and importing it), then deleting it once done - this should keep all your activity to that VM, and everything would be deleted with the VM. I'd also suggest using a 'portable' browser -like firefox, then deleting the whole folder (preferably securely, with something like eraser) if that's not an option. Portable apps are great for leaving a minimal trail on a system.