Nylon strings will work fine, as long as you can get some that will fit. Nylon has been used on classical guitars and ukuleles. They will NOT work with a magnetic pickup, so if she wants to shred either put metal strings on, or use a piezo pickup.
Welcome aboard, Mark! You will find lots of helpful folks and information here on all things CBG (CBU, CBB, etc.) Feel free to poke around, ask questions, and post your stuff (photos, recordings, videos, etc.).
Up in the Free Resources menu at the top, you will see a number of plans for building and playing cigar box guitars/ukuleles/banjos/dulcimers/violins. I encourage you to download any of the plans, take a look at them, and ask questions from there. Also our site is VERY searchable. I bet you your questions have already been asked, so take a few moments and look around.
Join our Chat sometime (see the bar at the bottom of your browser page). Keep in mind that tho the chat window says there are X number of people online, not all may have the chat window up.