I am a lefty, too, but the only thing that makes it different is many firearms are not ambi, so mag release takes a different method. I will state that you also should be shooting off-hand. Therefore, you should also have some understanding of the difference between left and right hand shooting. I shoot offhand as well. The same factors in grip for right hand apply to left hand. As to her not using both thumbs forward, that is not unusual, sometimes it is also due to size of hands to grip, other times it is just preference.
When shooting with two hands, the important part is getting your hands/palms on as much of the grip as possible, and locking the wrists. Turning the offhand wrist down, so the thumb is pointing straight forward locks the wrist and aids in recoil management. That, too, will get the thumbs aligned forward. Dry fire practice will help, as you can study the grip and see how steady the firearm is with one grip over the other. The coin on the slide is great practice for new shooters, regardless of grip style.
[Edit] I also feel that being a Lefty and being able to shoot with with either hand helps in training others, as I can have the person stand back safely to one side and see both sides of the grip/how I grip the firearm.