Fortified milk is just milk to which supplements are added. The quality of those supplements for food fortification is more difficult to control comparare to a supplement from a reputable retailer, so I would take the fortified food as an add-on, more than the sole source of minerals and vitamins, especially when it comes to B12.
Of all the milk alternatives, oat is the less nutritious. Soya has the highest protein content, almond has high calcium.
If she is taking calcium and vitamins D supplements, she also need vitamin K2 to make sure the calcium goes into bones and not arteries. K2 is harder to find in food compared to K1. The highest source is natto, fermented soya beans.
Once she becomes a vegan, she will have to supplement with B12, EPA-DHA (algae based omega 3, not the flax seeds, those provide ALA), and iodine unless she eats lots of seaweed.
Whole grains are a big part of vegan diets, so she will have to consider rice (white, red, black, brown, wild) , buckwheat, quinoa, polenta, chickpea flour. There is a lot of junk in the vegan substitutes world, so she shouldn't based her diet on the Frankenstein-food such as fake sausages, fake cheese, fake burger....
Soya sauce is used to marinate many tofu dishes, so gluten might be an issue in some shop pre-marinated tofu or tempeh.
I go to Asian supermarkets for my tempeh, tofu, natto, edamame, mushrooms, ginger and all the Asian vegetables.
Middle Eastern cooking books are a great inspiration for dishes . The Forks over knives website has some nice Tex-Mex recipes.
A vegan diet can be very healthy but requires some planning to avoid malnutrion. A good book is Becoming Vegan by Brenda Davis or get the bullet points from this 1h30 video.