Imagine someone walking into your home, dumping your jewelry, art and family heirlooms into a trash bag and throwing it in a dumpster out back.
That's sort of how a hoarder would feel if you came in and cleared out all that "trash" getting in the way of a healthier life. In order to even begin to help, you need to establish trust by making it clear you will not sneak in the middle of the night and clear the living room of all those newspapers. Compulsive hoarding is typically considered to be a component of an anxiety disorder, in which case the behavior of collecting, and keeping, all of that stuff is helping your friend to feel safe. If he or she thinks you're going to destroy that lifeline, you won't get anywhere. And if you do destroy it, the first thing your loved one will do is start collecting again.
So approach the situation with caution and compassion. Rather than attacking the hoarding behavior, try to understand it. There's a misguided but intricate logic to this coping mechanism, and respecting that is the only way to even begin to help.
Then, you can start making your arguments for change…