Having to teach people how to play from scratch can be a bit challenging once you've been playing for awhile. The tutorial is great for learning basic mechanics but LoL has a lot of nuanced things that they don't cover in it. I noticed that a lot of the things I do (creep score, last hitting, warding etc) I just do on auto pilot now so it was hard to remember that going into the game I had to be very aware of the fact that even small things that I don't think about are vital. I recently tried to teach two of my friends how to play recently (they never played any MOBAs before, only FPS games) and it was such a rewarding feeling watching them climb to silver!

What I would recommend is to read a beginners guide yourself so that way you can better articulate the things that she needs to focus on at first without risking throwing her into the game with an overwhelming amount of small gameplay stuff you get better at the longer you play. I would also recommend laning with her bottom lane at first that way there's some security and you can watch and help more effectively. Also I found it was better letting my friends pick a main at first (a champ that interests her but isn't too complex) that way they can focus on learning how to lane without constantly having to relearn new abilities.

It might take a few games and some time, League is one of the few games I feel like people feel helpless in at first. There's a lot of info to take in and early game blind pick can feel like a gamble but the more time she invests in playing at first the better her chances at really enjoying it.