I've been working with 'hoarders' for over thirty years, both as a cleaner, and a de-hoarder.
I grew up in a squalor hoard, and hold a viewpoint based on watching a parent descend into a self made hell, taking their children with them. It didn't end well for anyone.
It's a deeply destructive and heart breaking MH condition, but the hoarding is actually the SYMPTOM not the actual problem.
Which is why trying to remove, hide, solve, store, dissipate, the symptom, doesn't work and is counter intuitive to the person even when they know nothing of their own condition.
There's some nasty judgmental stuff on this thread that just further stigmatizes and 'others' sufferers.
People with the disorder come in many types.
Yes some can be angry and mean and all sorts of negative emotions can be present, sometimes through character, sometimes as a learnt defense mechanism, but many are desperate, hurting, humiliated, and some so ashamed that taking their life seems the only solution.
Nearly all 'types' have a traumatic back story or five buried under everything else, but the essential issue that all have in common is the same: development of an emotional attachment to possessions at a level that enforced parting causes anything between deep psychological distress and damage, to life ending being genuinely preferable.
You can't fix this, and in the nicest possible way, don't have the skills and knowledge to help your friend, which is why you are suffering, and they continue to be delusional and unreasonable in the face of your efforts and intentions.
TBH ime most of the professional help via authorities out there is pretty useless as much of SS, NHS, FB, have little idea/desire how to help the person, as they come at it as the symptom being the big issue to deal with, and want to focus on that for the benefit of others, rather than a holistic approach for the benefit of the individual, and much of the experienced help is outrageously expensive and only some are even good.
There are a handful of good charitable and 'at cost only' organisations out there, sparsely scattered. See if there's anything near you.
Your friend actually needs many sessions of identifying certain things individual to each person and their situation, then an individual realistic plan making with them, help with collating, sorting, help with developing decision making strategies, help with developing volume and spacial perception and awareness, and much discussion to help change how they currently perceive and value things, space, relationships, themselves, the past and the future, along with the time and space in between, to absorb, fight, mourn. collapse, come to terms with some of it, stand up again etc. All time your friend probably doesn't have.
How far and how fast someone can be reached, and how much progress can be made depends entirely on where they already are in self recognition, how desperate, and what they really want in life. (not always what they appear to want)
Many people I've worked with, actually have more insight and recognition than they show to family and friends. Denial can be a useful defense position.
However there's a phase of the disorder where there's no recognition at all and this can last decades, according to situation, finances, self reflection and knowledge and your friend could equally be at that point, in which case you're the problem in their mind.
Having taken in the clothes, the best thing you can do for them (rather than yourself - which would be return them) is vacuum pack them into boxes of the same size, measure the space they take up and set a date that you'll be shifting the 'X foot by X foot cube' to their new or current home. State it, and write it large on an envelope, stick it by their front door, and do it.
The alternative is to recognize that for many being safely separated can help loosen the bonds, and store them for now before returning them in batches, helping to sort as they go. Riskier, if necessary groundwork hasn't been done.
I fear the way things are going, your friend is set to loose both their current home, and the one they expected to move to. Helping them in whatever ways you can to prevent that, and being there if it does, may be the best thing you can do for them as a friend at this moment.