I'm going to echo the comment about yeast. It's really valuable!!
I would say try to be supportive of the whole thing but don't be invasive - maybe encourage her to start slowly and learn as she goes?
It's notable that when you first go vegan there can be a sort of period of time where it doesn't feel that good. Like I got stomach cramps and stuff for a couple days. It's pretty normal and after about two weeks maximum I felt totally ok and since I went vegan I actually feel more energized and generally healthier.
Re: the comment about throwing away all leather/wool etc. - this is nonsense. Any clothes or products she already has can't exactly be given back - it makes sense to use them up and just not buy any more.
In terms of skincare and stuff there are lots of good options and she doesn't have to feel limited. Lush has lots of good vegan stuff and google is your friend.
The main of thing is food of course. You'll both get better at preparing it. There are lots of little tricks you'll have to pick up but it's amazing what you can do with no animal products. My boyfriend and I swear by the Pure brand soy butter which lasts for ages and tastes exactly like the real deal. Soy milk can be horrible at first but you can wean yourself onto it (it's worth doing because it's really good for you. I started with strawberry flavour). I also second the comments about picking up supplements because they are pretty necessary at least until she gets into the swing of thngs. I take B12 and a multivitamin on alternating days.
I know this isn't super important but if she does it she's going to want some snack foods (like sweet things, unhealthy ones, there are plenty of cake recipes and stuff online that are good but if she goes to like a sleepover for example and all her friends have snacks then it'd be a shame if she didn't get to join in) and the ones I like that are vegan are jammie dodgers, bourbon creams (the cheap kind, usually), oreos, skittles, crisps (sometimes, not always.)
A good rule of thumb is the cheaper it is, the more likely it is to be vegan. You'll need to stock up on staples - you can do a lot with pasta, rice, lentils, all that stuff. Most importantly make sure she eats enough to maintain a healthy weight. Lots of people initially lose a bit of weight.
Anyway I'll stop now. Good luck to you both!!