Gonna agree with this and Chris too. This is a big part of it though--I firmly believe weight training to be the fastest way to transform your physique weight loss wise aside from diet considerations. Of course you can't lose fat without dieting :).
However--what matters the very very very most in the initial stages of fat loss for someone not used to training at all is this:
1) doing physically active things they ENJOY doing constantly. It won't matter how effective weight training is if she gives up. This is where Badger comes in with yoga, tennis, hiking etc.
2) doing as many different kinds of activity as often as possible--i.e. spending most of EVERY (or most days) day active in some random fashion or other, even if it's just walking, biking, or whatever. Again this is where Badger comes in with his comment above. Pick up active hobbies like hiking, or bicycling, or whatever. Simply being outside at all has a beneficial effect.
3) sticking with it when it gets hard. You can't enjoy every day at the gym or cooking healthy food, so the positive encouragement and focus to stick through the hard transition is essential. Again, this is where a variety of activities come in: great to have a perfect plan, but some crappy days she doesn't feel like the gym she wants to go swim or something....great! Something is always better than nothing and if you enjoy it you'll keep at it.
4) doing things together when she wants you to. Obviously can't be pushing her or she'll start to resent it, but if she wants to have you in on it, might be best to do it even if it's not in your plan :). Also difference between asking suggesting and pushing (obviously). if you feel like doing some swimming ask her if she'd like to go to the pool with you. Or the trail. whatever. Make it "alone time" instead of exercise. Team stuff enjoying nature or whatever. Less she thinks exercise and more she thinks "fun" the better.