Best tip is don’t push her, Buy her a basic, well fitting bike (which you already have done) and invite her on any ride you go out on, be honest about what your doing and how tough you think she’ll find it, be prepared for her not to share your level of enthusiasm for bikes…
If she comes out with you be extra patient, encouraging and don’t assume that the stuff you find simple will be the same for her, calmly talk her through stuff before tackling it, take regular breaks and talk about how she’s getting on, if she feels harassed, bullied or like she’s holding you up then she’ll probably just jack it in, Oh and you carry all the kit, shoving a pack full of spares, tools, food, water and waterproofs on her back will not aid her confidence…
My missus loves tow path bumbling between pubs in nice weather, but she hasn’t got the confidence or skills to do trail centres or anything at all technical, she got very upset with me when I took her to Swinley as I said it was “flat”, which it is relatively speaking, but as she hadn’t been on a bike for the best part of 20 years it was a touch beyond her.
She’s since done a couple of charity bike rides and some more adventurous pub trips but to be honest I doubt she’ll ever progress much beyond that, she’s keen to do family rides with the nipper once she’s old enough but “proper” mountain biking just isn’t her bag, she’s tried but it’s not for her, and that’s fine…