Pretty much repeating what's been said here, but a) how does she actually use it beyond her own posting behaviour, and b) who is she talking to?

If she literally just posts random crap on her feed and talks only to kin, a concentrated effort to move family to something like Friendica might be a useful project. If she's interacting with a wider network of friends/acquaintances, community/hobbyist groups, news outlets (you can do that in the States, can't do it in Canada anymore), Facebook games, and various slop vectors, you have a much more challenging task on your hands.

There's a lot of shit on Facebook that's hard to replace in a way people who otherwise don't care about Meta would use. If you're looking to start anywhere, I'd say start setting up a Friendica instance, have a plan to onboard key family members with the least amount of friction possible, seed your own feed with grandma bait, and keep bringing it up ("Oh yeah, [x] was great, I took a lot of pictures, here's the link to my Friendica page. Oh, you need an account? It's just your e-mail, password's GrammyGram@420. I'll show you how to change it*.")

*Not a Friendica user so I dunno how it works in detail, but you get the idea. Also doesn't need to be Friendica specifically, but this one's usually brought up when talking Facebook alternatives specifically.