I'm a lefty and one can adapt, especially if they start young. I agree that eye dominance is key. As it turns out, I'm right eye dominant and no one noticed. It would have been much easier if someone had taught me to shoot with my dominant eye.

Another possibility is a modified military gun from around WWI-WWII that has the old straight bolt (I recommend a Mauser). A lefty can manage these. A scout scope can be used for optics, mounted on the rear sight base, or install a peep sight (look at Williams, Lyman or Mojo sights).

Get a good single shot rifle like a Contender or Encore or even a less expensive H&R and then it won't matter which shoots it..

As a lefty, I can attest to the challenges of using a RH rifle. But with practice using one becomes second nature. But LH versions are much nicer. And my suggestion goes along with the earlier post. Soon or later if both hunt you will need to buy them each a rifle anyway. So why not save up and give each one their own rifle. Just don't go too cheap and get something like a Savage Axis or Rem 770 or other 'el cheapo' model. And let them pick out what they want (within your budget). That way they will like it an feel more confident and prouder with it.