One of the ladies in our quilting group sometimes brings her mother on our retreats. Her mother is either in her 80's or possibly early 90's but she pieced a rather intricate wall hanging at the last retreat that she attended. Although she knew how to sew, she'd never pieced anything before.
If you're going to teach her, start with teaching her how to use the tools she'll be needing---rotary cutter, rulers, mats, etc., even if it's just watching you do most of it. Also, allow her to help in the selection of pattern. Simple is one thing, boring is another. If she's bored with the pattern or fabrics she'll not get as much enjoyment out of it.