Reddit reviews One Thing at a Time: 100 Simple Ways to Live Clutter-Free Every Day

Apologize to her for trying to push her to get rid of things too quickly (hint: if you work with her on "cleaning" or "decorating" or "decluttering" these will also result, as you go through things together, in at first one or two items and then as she gets more confident boxes/bags of items that she is willing to have go to another "home" ie: to donate or get rid of)

ask her what kind of help she wants

Do the help that she wants. Better with her than for her.

Browse amazon online with her looking for books along the lines of what she wants help with. Helpful hint for her: don't buy everything, avoid the "buy together" promos like the plague, "want list" any items of interest and then narrow it down to two or three when buying. Work with her to set up a want list if she doesn't have one already.

Also "just happen to" browse clutter and hoarding books in amazon if she hasn't already done so above. She can add some of those to her want list.

Come across this book : One-Thing-At-Time-100 Ways to Live Clutter-Free and include this in the two or three books she actually orders.

On your way out, offer to take the items she has set aside for donation/trash to their new destination. Even if it's just one or two items. Respectfully do what she wants with these things, and report same to her next time you connect. hint:Trust will build.

tldr: understand and value her priorities, help her establish the pathways to achieve . Provide logistical support, but don't override her choices.