Hoarders have to want to change. First, they have to see that what they are doing isn't okay, and that it is affecting their quality of life/ability to socialize/etc.
Once she does that, you can see about therapy and getting her to let go of stuff.
Honestly if my partner was hoarding I would approach it as an organization project. I actually am the one with the hoarding tendencies, so I keep it in check out of my own interests. If I saw nothing wrong with hoarding then I'd never change, obviously. So that first, approach it as a together project, a partnership, where you discuss the purpose of each item and if you truly need/want/love it.
If she truly believes she needs 40 pairs of shoes(friend of mine truly believed he needed 100 beer cozies--and he didn't even drink beer!) Then arrange them in front of her and ask her how many pairs of them she honestly wears in 3 months. I bet the number is closer to 5-10 pairs. I have worn the same single pair for months out of laziness before--but the reality is, you don't need 40 pairs of crocs. She needs to see that and remember that she does have other pairs of shoes she can wear instead.
The main thing besides purging is organization.
A psychologist recently found that for those who hoard, they either forget they have an item or are only able to visually process things. I.e. a clothes dresser will be fully empty but the shirts will all be on top.
I have a friend with ADHD and he also has a focus on visually processing items. So for clothes I recently re did his closet--everything that he uses daily is directly visible and easy to reach. Everything that isn't used often is near the back. Shoes have a spot. Dirty laundry has a hamper. Half-worn clothes go on a hook. Cardigans, scarves, belts, etc. All have their places.
This means that he doesn't have to decide where to put things(getting caught up in the decisions happens to both hoarders and those with ADHD, and sometimes severe ADHD leads to hoarding) so I took away those decisions. Belts always go on the belt rack. Full stop. If you have too many belts for the belt rack, you cull the belt herd by getting rid of the ones you wear least.
It takes time and most importantly, a willingness to change behaviors, but if she is willing she can keep it under control. The important part is willingness. As long as you both approach it as a behavior pattern and an illness, rather than just "okay let's get rid of junk." Then you'll make a lot more progress.
Does she have any hobbies besides shopping/acquiring things?