If neither of you want her to lose any length, what is *her* motivation for talking about cutting it?

There's an abrupt change in her hair at the bottom of her shorts. I thought natural thinning at terminal length was usually more gradual. Was that thinning present 2 years ago at the ends or has it developed since? Did her health abruptly change (for the better) about 6 years ago? Or did she stop coloring or otherwise damaging her hair about 6 years ago?

If your joint goal is more length and you subscribe to the theory that split ends spread and get worse (not everyone believes that), then a trim above the bottom of her shorts seems in order.

If you're more comfortable doing that in a few smaller steps over several months, ok, but that delays getting rid of the ragged ends. Unless she wears her hair down in public which is unusal at her length, nobody but you two will know if she cuts a bunch at once or in small stages.

I agree with the others that it's better for you to do it than a salon. Even if you are nervous, you and she are the ones most invested in her hair, and you won't "accidentally" go short because she "needed" as could happen at a salon. Once you are both used to the process, you can trim an inch or so 2-3 times a year to maintain the ends while growing.

She has beautiful hair; you're lucky no matter what she does.