Thank you for your question. I will try to assist you in answering it today but please feel free to ask as many follow up questions as you like until you are 100% satisfied.
An engagement ring is considered a conditional gift. In other words, the condition for the receiver of the ring to become the owner of the ring is that he/she will marry the giver of the ring.
In the event that the condition is not met, then the ring must be returned. The general rule/principle is that all gifts given during an engagement (with the exception of small tokens of affection) must be returned in the event that the engagement is terminated by mutual consent.
Now, there is an argument to be made that, should the man (in this case) have terminated the engagement (ie. making it impossible for the woman to meet the condition of marriage) that he will forfeit the condition and that she will be entitled to keep the ring. This is however a question of fact as to who is to blame for the termination of the engagement. I note that you wrote that he "packed his stuff and moved out" - but the question will be what lead to this event? Why did he move out and what caused it.
I cannot comment or be the judge of that, but that will determine whether she has any legitimate claim to the ring or not.
I hope this answered your question.
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