I was in the same situation as your girlfriend when I turned 30: Hadn't ridden a bike since age 17. I thought it would take me a while to get back into it, but it took no time at all. I didn't have to "re-learn" it or anything; the skill was still there after all those years. Couple of months later, I began cycling for transportation in Boston, on my own, and having a great time.
Getting on a comfortable, upright stepthrough bicycle was key for me and it looks like you built up just that for your girlfriend. I think it's the right way to go. And yes, I had to keep one foot fully on the ground when I just started - no big deal, and I gradually raised my saddle higher and higher as I learned to balance better and got more comfortable with the bike.
Good luck to you both and have fun!