I wouldn't deem to know the law on this - and I bet two lawyers would give two different peices of advice.
I really cannot imagine that if the remaining tenant (R) voluntairly vacates and everything is done properly regarding keys etc that legally you'd have a problem with absent tenant(A).
If you have a signed lease and the only signatory was A then,possibly, there'd be a legal problem if A turns up the next day or so. I still doubt it, but it would be better if both had signed or even better just R. (Not that there's any obligation to have written contracts).
If you and R sign a returned key receipt,copy for each of you, that's great.
if R signs nothing and just throws the keys back, or not at all, then that's slightly problematic. But ,again, I doubt there'll be a legal problem.
It's best ,of course, to be as nice and friendly as possible in order to get that signature.
I can't imagine R signing a note that states "on behalf of A and R I am returning keys to the LL as we are vacating XYZ". (I can't suggest a bribe to obtain that)
Has A left stuff there ? Again ,let R sign for it and take it. If R refuses then ,again small problem. A 's possessions must be carefully stored.
To sum up, if R goes, even without a signature I'd still go ahead and change the locks etc.
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Now, someone else, an expert in law, may say that you should contact A and prove that you have tried to do so. Do you know where he is ?
Anyone can be a witness, but using one with no connection to you would be wiser than using a relative.
Nobody can use any weapon or force to defend "property". Do you mean defend yourself?
Yes,of course. But this brings up a question....
.. is A some presently in jail or mental hospital and you fear for your personal safety ?
Your query suggests a fear of damage to property and/or harm to yourself as much as any tenancy dispute, in which case I don't know how anyone here could or should advise you properly.