Well, I'm personally not a hoarder, but I do have OCD. I definitely wouldn't want to be approached with no proof...like, if someone came up to me and said "You have a problem. And here's what you need to do to fix it."

Perhaps you could find a video on hoarding that you could watch with your friend? Maybe your friend would recognize that she shares some of the symptoms with the people shown on the video. Also, make it clear that you are approaching the subject because you care about her very much, and you want the best for her and her animals/things.

If she reacts badly to your positive gesture, then that would be too bad, but it wouldn't be your fault. In the end, the person with the problem must decide to help themselves.