If it doesn't work out for any reason, will the winshit viruses still be
there if some idiot puts winshit on the hardware?
If you let Trisquel take the whole disk, no: the installer will format all
the partitions.
What are some good libre programs that she might enjoy and find useful?
Shotwell is good enough for me so that's all I know.
Shotwell is good at organizing pictures. GIMP is installed by default: it is
a good at editing pictures... but requires learning.
Is it bad manners to plug her phone into my Belenos machine before I do
anything to make sure there won't be any problems getting her pictures?
As long as you ask her permission, I do not think so.