Etiquette, shmetiquette. What we do in the US is different than in English, for example. And there are fewer and fewer manners all around.
I think it's more important that she avoid the unpleasant behaviors, like those mentioned below - slurping, not holding over, bringing mouth to soup instead of soup to mouth, talking with a full mouth, what to do with her napkin, how to handle foods she doesn't like, and so on. Place settings are designed for right handed people, dating back to when people thought there was something wrong with lefties. And they stayed that way due to the majority of people being righties. Schools also used to force lefties to write with the right hand, but I think we've progressed past that. So should we with utensils.
I think you should focus on skills, not which hands she uses. Show her how you do it (so she can learn the relative position of knife & fork, how to cut without sending the food off the plate, and so on), but say she might like to do it the opposite way as she is a lefty. Remind her that not all lefties do it the same way anyhow. Let her decide what feels right for her.
Control of her food, and knife safety, matter more. Also, I know many lefties who ask to sit at the left end of a table so they aren't banging elbows with the righty next to them, and I'd suggest you encourage/allow that, especially at a crowded table. That may mean educating some relatives.