He has done his due diligence with multiple voicemails and emails.
That's simply not how the law works here. She is not required to maintain contact with him, his due diligence in contacting her is
completely irrelevant. At this point he can't do anything. When she get's five days past due he can post an eviction notice, then 29 days latter he can have law enforcement evict and change the locks.
That is the very soonest he can get the place back legally unless he gets in contact with her.
He has to protect his property. If he is available to let her in if she calls, and she already has his contact info, she won't have any cause for damages.
Protect his property from what? She is allowed to give keys to her residence to whomever she pleases, if someone gains entry with her key and causes damages she will be liable. But at this point no damage has been done and she is current on rent.
The advice you are giving is wrong, and could be very costly for the OP.