One additional thing to take into consideration is the hoarder's future ability to find the things that have been moved into storage and his/her ability to retrieve them. While it's quite true that a great amount of what we hoarders have is not being used and is not "important" by the standards of others, much of it is being used. We often don't put things where you, the non-hoarder, would logically put things. A car title might be in a folder inside a laptop case inside a cardboard box that is on the floor underneath our dining room table. The key to our safe deposit box might likewise be in a (different) odd location, yet we ourselves may know exactly at this moment where to go and find it when we need it. Granted, we may have misplaced many things, but there are often many, many important things that we know exactly where to find . . . until someone else moves it, that is. This is one reason that many of us start "freaking out" when someone else suddenly starts moving (and especially trashing) our stuff. We may not have the words available to us at the moment to say out loud to you, "Hey, my car title is in there!" (Don't spend a lot of time here trying to figure out why the person has a safe deposit box but the car title isn't in it. Just accept that fact for the moment: for hoarders, lots of important things end up in places that would seem to others to be highly illogical.)
This is one reason that the "mutual agreement" that others mention can be so important. In trying to help us out with one problem, you may inadvertently be creating lots more problems for us if we're not involved in the process.
It sounds like you are truly trying to help; the fact that you are even asking these questions makes it obvious to me that you care about the hoarder in your life. You seem to want to make sure that the person can still "touch" the items, so the following comments about boxes probably don't apply to you and the way you're planning to approach this. I think I'll go ahead and make the comments, anyway. It could be helpful to you or to someone else in the future.
Another potential problem has to do with how things are stored in the storage building. For example, if you were to box up what I own and were to stack boxes in that storage place but I can't lift the boxes, then that, too, could pose a problem. I have had back problems at times, and something that might be easy for you to lift might be hard for me. Once again, the involvement of the hoarder in the "moving the stuff" process can be important here. Just my thoughts.