DVDs are dirt cheap, plentiful as fuck, don't have DRM bullshit to have to deal with, last for decades when stored properly, and still look pretty damn good with deinterlacing. Plus, they don't run any of the risks associated with piracy. Am I allowed to copy my DVDs onto my hard drive? That may be a legal gray area. But can they see that I copied my DVDs to my hard drive? Of course not. And I'm not making my ISOs and MKVs available to the world for download.
Spend 4 bucks on a used DVD. Give her the ol'
dd if=sr0 of=~/Videos/Movies/Title.iso
And keep the disc for basically forever. Copy it again if something happens to your file. EZPZ. Plus, it's cool to own a physical thing imo.
One last thing: DVDs come with subtitles. I have a hard time understanding spoken words. I like to read my movies as I watch them. Makes it easier to know what's going on without cranking the volume to 11. Speaking of which, the menu for the Spinal Tap DVD is excellent.