If you *are* going to risk it with a machine, definitely use a zig-zag or stretch stitch. I'd just make sure and practice on a swatch first. Also, you can put a piece of paper over the knitting to help it glide between the feet without snagging. It rips right off when you're done...although, that said, that works better with a straight stitch. You might have a
moment trying to rip paper away from a zig-zag stitch.
Guess I'm just brainstorming, here!
ETA: After more thought, I'd say she'll be happier with the finished product if she doesn't add a lining at all. I think the knitting will stretch, and pull away from the lining--there's just no way for it to lie in the way that two pieces of fabric will.