My wife is for 2 months now after watching What The Health. No real difference in energy, weight, vitality really. She's stuck with it though, props to her resolve.
Other documentaries on Netflix (mainly Vegucated) is how I first got interested, too, but What The Health is one of the newer and better ones.
What does your wife eat? If she was eating relatively healthy before, or if her vegan diet now mostly consists of processed foods - it's surprisingly easy to fit in a lot of sugary vegan goods, which is why I don't bake often - it may not make an impact.
For me it was a fairly immediate impact in energy level, weight loss, and... bowel health (all that fiber...I wish someone told me to introduce the higher fiber vegetables, legumes, and fruit into my diet slowly ). Longer term I found it helped with my asthma (never use an inhaler now, though regular excercise also helped with that) and with reducing frequency and intensity of migraines (though I also tend to avoid my trigger, anyway).
Not that I'm advocating the vegan diet as the "perfect" diet. Actually, I'd say people who aren't interested in researching nutrition and the vegan diet shouldn't do it. You at least need to be aware of certain deficiency risks, mainly B12. On the whole, though, there is plenty of research into vegan diets being healthier than the "average" diet, but there are other balanced diets out there (Mediterranean being one of the most researched) being excellent. So I'm not knocking other people's personal choices when it comes to diets. As with most things, it's a YMMV.