If a person voluntarily moves out of a residence, can they come back anytime they want because they weren't evicted?

This is not as easy a question to give guidance on as one would think.  Are the two people married?  Are they both on the lease?  Has she continued to pay rent?  If she is on the lease and continues to pay rent then she can continue to have access to the apartment.  If you were married and started a divorce proceeding then you would ask a judge for exclusive occupancy at this point in time.  But the only person who can evict is the landlord.  You could ask the landlord to issue a new lease only to you but she has the right to continue under the old lease.  Seek legal help here.  Good luck.