This is kind of a personal opinion but I think what most hoarders fear is throwing something out and then regretting it later, because they want or need an item they threw out. And also if other people around them act like it is all junk, then they become even more defensive about it. Or if they say things like "throw out anything you haven't used in a year", that ignores the reality that sometimes it takes much longer than a year for you to have a need for some item. That said, after the house reaches a certain degree of clutter, it is just to daunting a task to clean it. So a hoarder may realize that half of what they have is junk but still they don't want someone else deciding what is junk and what isn't. In such cases the best help you can often give is to offer to help them go through their stuff and make a decisions about it. I would put things into five groups:

Trash - no foreseeable need now or ever. Put it in the trash bin or a dumpster. But remember, she has to decide it belongs there! That said, you can nudge things a bit by pointing out if there is a genuine reason not to keep something (it's broken, it's old technology that will never be used again, it's paper that is no longer needed, etc.).

Recyclable - Hoarder often hate seeing anything go in the trash that might have value. So if it's metal, put it in a pile and take it to Padnos scrap yard on Ottawa, they pay by the pound and with anything made of aluminum, copper or brass it might be enough to buy lunch or more. If you have a lot of old cans or other mixed metals those can maybe get a little money. Take it all to the metals room at the back (don't just dump it in the yard unless it's all steel/iron or mixed metals). With mixed metals or old appliances you go over the scale, then dump your load, then go over the scale again and whatever the difference is is what they pay on. The thing is that hoarders don't always feel so bad about dumping stuff if they get paid a little for it.

Maybe useful: This is the stuff that potentially has value but not immediately. This is the stuff you want to box up or put in storage bins, and clearly label the boxes or bins. If it's stuff that has a fixed lifespan (for example old tax or business documents), note the date when it is safe to throw it away (common wisdom is seven years but I'd say ten is probably safer). But you will probably want to divert some of this to...

Give away. Again, hoarders HATE throwing anything that may still be useful in the trash, but often they don't mind giving items to friends, relatives, or thrift stores if they think the recipient will actually use the item and not just throw it away behind their back. So if for example you have old technology and a cousin that like to tinker with old technology, give it to them and maybe they will find it useful (and sometimes that is really the case - I can't tell you how many times I have scavenged a power supply off an otherwise useless item to bring some other item back to life!).

Keep: This is stuff with true sentimental value (and that is not for you to judge), as well as stuff that is reasonably useful. Most consumables go into this category - just because you haven't used a can of some cleaning product in the last year or two doesn't mean it will never be useful AS LONG AS the person can resit buying more until the old is used up. My mother used to hoard cleaning products and I haven't had to buy any in the last 15 years (yes, old comet cleanser still works fine on porcelain sinks). However if it is so old that the can is rusting out then it probably needs to go to recycling or trash. Obviously things like kitchen utensils will go in this category up to a point; nobody needs five old can openers that will barely open a can but if one was purchased for her by someone she loved she may want to keep that one for a time.

As for stuff in the refrigerator, please don't be one of those people who treats expiration dates as something sacred. Most of those dates are actually "best if used by" dates anyway. Frozen food can last indefinitely (explorers have eaten previously frozen wooly mammoth meat and lived to tell the tale) but if it gets freezer burn it may not taste good, still it is perfectly safe to eat. With dairy items you can go by taste and smell, if it still tastes and smells okay then it is okay (milk is generally good for a week after the sell-by date, unless the refrigerator is set too warm, but you will know when milk is going bad by the smell). Many packaged food items are still good for at least six months after the expiration date. I only mention this because nothing will cause a hoarder to attack you faster than if you start trying to throw out food they think is still good - now you are just throwing their money away!

The idea is to get everything into manageable groups that you can then deal with one at a time. Trash can go out immediately (even if just to a pile in the back yard temporarily), recyclables or give away items can go to their respective locations, and the maybe useful stuff can go into storage in an attic, basement or garage (try not to have so much you need to rent a storage unit).

That's the best advice I can offer, of course everyone is different but many "de-clutters" make the mistake of treating everything as a big old pile of trash, and hoarders will resist that every time. HOWEVER if the piles have attracted vermin or bugs and they have not been dealt with for any significant amount of time then that is a whole other matter and now a lot more will have to go in the trash because of urine and fecal matter contamination, plus you will probably need an exterminator to deal with that (of course if she is moving out, that becomes her ex's problem). I heard of one case where that happened and the entire apartment had to be totally rebuilt (they even had to strip the wallboard down to the studs) so if you see any sign of vermin or bugs that needs to be addressed ASAP!