Under what terms was she storing it? Was there an agreement in place to store her personal items? What are the other details? I would never recommend destroying the property. But, without more details, its difficult to provide a more thorough answer. In a trespass situation where someone comes on your property without your permission and leaves something at your house or in your yard, you can probably just remove it. If there is some type of agreement to store some property, it becomes a little more complicated.
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I agree with the other attorney. One thing you can do is either go into your local police station or call their non-emergency number and ask if they can inform you what your local law is on this issue.
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This answer assumes that you did not have any particular agreement with your friend about how long you were going to keep the property or under what terms. If those facts are true, then you have an at-will arrangement with your friend to take care of the friend's stuff, which you can end at any time, although you have a duty to exercise reasonable care towards it while you have it and while you get rid of it. One possible alternative is to dump the stuff on your friend's porch, but depending on the circumstances that may or may not be taking reasonable care. If it were me, and I really wanted to be careful, I would seriously consider following this procedure, although there are some risks and there are some definite costs:
(1) Take an inventory of the stuff so you have clean records. Also keep a written log of everything you do after this point.
(2) Give your friend a deadline (30 days is always a good round number), and tell them that after that deadline, if they haven't gotten their stuff, you will order a storage unit, place the items in the unit, and pay for only one month's storage rent. Make sure the notice is in some kind of written form (like email or mail) and that you have confirmed that the friend got the notice (certified mail is best, but calling and confirming after sending an email is not bad if you keep a log; if you were being extra anal, you could pay the constable to serve it on them).
(3) When you rent the storage unit, make sure you instruct them in writing that your friend (by name) can pick the things up. Write down the name of the person you gave the instructions to, date, time, etc. Give your friend written notice with clear instructions as to where and how to pick it up, and the name of whoever you talked to, and a copy of your written instruction you gave to that person to let the friend pick things up. Send it in the same manner as before, making sure you know they got notice.
(4) The storage unit will have its own means of disposing of things, prescribed by a combination of law and contract. The only downside is that you may end up owing the storage place money; keep in contact with the place so you can make sure they do their job as efficiently as possible.
Just to be cautious, you may also want to read the instructions in Utah Code 78B-6-816, which is what landlords are required to follow when a tenant moves out and leaves their stuff in the apartment. This procedure is loosely based on the ideas in that section.