I'm very strongly left handed but learnt the guitar the right-handed way. I thought that actually this was easier, because you need lots of strength and coordination in the left hand. I reckoned that I made progess faster than if I had learnt right handed.
But...one day I mentioned this to my guitar teacher.
He said that yes, in the early stages it was probably an advantage, but that I would never be a virtuoso player
because in the end the music is made by the right hand, and that to inject real musicality into your playing you need to make the sound with your preferred hand.
(I wasn't sure whether to believe him, but I noticed that if I pretend to play a violin I hold the imaginary bow in my left hand. This is despite all those years with a guitar where the left hand was in charge of the strings).
He added that, although I'd been playing for around 3 years at the time, if I were really serious it would still benefit me to relearn left-handed. Since I'd only taken it up at age 16 and didn't have any ambitions beyond getting a reasonable tune out of it (I was around grade 4 level) I didn't think it worth it.
So the point of this ramble is that if your friend wants to play for fun, have a lot of choice of instrument and make reasonable progress in the early stages, then try to learn right-handed. If they want to experiment they could always get a standard guitar and restring it. This would give a taste for how it feels that way. ("Starter" guitars are not expensive compared with, say, a violin, so this would not be too much of an outlay).
Then, if they were really serious and felt that playing left handed gave them an edge they should take the plunge and get a properly adapted guitar and go for it, taking into account comments from the other posters about choice, ability to pick up other instruments, etc.
Teaching should be no problem - the teacher can sit opposite and demonstrate as if in a mirror.
The only thing I would absolutely not consider is playing a right-hand strung guitar upside-down. Maybe Jimi Hendrix could do it but it would make even some of my easiest classical pieces practically impossible to play.