I might be mostly vegetarian and eat eggs and cheese and fish once a week, but I usually tell folks to reduce their eat. Sometimes I say instead to make sure they eat enough fruit and veg and whole grain. I say this stuff because it is easier for most folks to concentrate on things like this to improve overall diet without feeling like they have to spend days figuring things out.
Also, it's worth noting that the difficulty of going vegan varies greatly with your location. For example, eating vegan in Japan is a legitimate pain in the ass.
There are lots of reasons a person can not have a period. Sure, you can be malnourished, but I'm guessing the more likely contributor was not having enough body fat. IF a woman's body fat is too low, she stops having periods. It happens in a broad cross section of people. A person with an eating disorder will do it as will a woman that works out a lot and eats healthily. Coincidentally, weighing too much can do it as well.
I'm guessing a nutrient deficiency can do it, but I'm unsure on that one. It can be a hormonal imbalance too!
And there are a bunch of other reasons as well. It isn't even necessarily harmful - after all, people quit having periods when they are on birth control with no ill effects.
It's not, it's mildly tricky, and there's not much evidence that people following a vegan diet are nutritionally worse off than the people who don't think about their food at all, many of whom don't eat any fresh fruit or veg across a week.