Try adding her left and right index fingers. They’ve often more convenient to use, in any case.
Shut down the iPad completely after you’ve taught Touch ID — and you'll see the passcode prompt on a restart. See if that’s her situation or issue. If so, have her write it down on a 3 x 5 card to keep in a secure place.
<soap box on>
As to lots of baby boomers struggling with tech… that's an overused meme! Many do, but many of all ages do. And, many don't.
I'm sensitive to such remarks, perhaps overly so, because I had relatives in their '90s who were regularly emailing, downloading, etc. For sure, some struggled with certain aspects, but others were quite adept. (So, please don't this digression the wrong way!)
I also know baby boomers who are or were astrophysicists, engineers, accountants, historians, news reporters, programmers, etc. and they have no problem with tech. Indeed, they use it regularly in their professions and are often having to teach “youngsters” the tricks of the trade!
No doubt we have a fair number of baby boomers here at MacRumors, too!
<soap box off>