We've been recommended Total Yoga Collection, featuring Tracey Rich and Ganga White, by our yoga teacher for stuff to do over the Christmas break when there are no classes. We haven't seen it, so I can't really make a recommendation, but I think our teacher knows what she's talking about.
Have you thought about a trial class instead.
Learning yoga etc from a book/dvd can be trickier since it only requires small variations for something to change from being useful to either useless or actively damaging.
Ashtanga Yoga by David Swensen is good (though some of the positions that he gets into are somewhat alarming). He gives different ways of doing the postures according to how flexible you are, which is helpful. Wouldn't recommend doing yoga from a book (or DVD) without having gone to a few classes, though (which your wife may of course well have done), as alignment is all-important...
As said above, yoga really is not the thing to learn from a dvd...i speak as a practising yoga teacher.
Buy her a beginners course at a local yoga centre if available.
Iyengar yoga if you have a choice, or maybe anusara. Both types of yoga very strong on alignment.
Classes are best by far, but be prepared to search a bit for a teacher or style you like - most good teachers will give the first class free or for a reduced fee, no obligations. That said, you'll need to give it a few goes before deciding if yoga is for you or not. Theres a knack to the mindset as much as (more than!) the postures. Dont write it off first go, easy done with a duff teacher or a style that doesnt suit.
Re: DVDs - A humungous number of teachers have a you tube channel or clips these days. Worth a look before committing to a DVD (if only to check their accent/style of teaching doesnt grate!)