Within our common law there is a principal that states that an engagement ring is part of a contract, or promise to marry.  The ring is consideration in that contract.  If a man asks a woman to marry him, he will give her a ring.  If she accepts the engagement she keeps the ring.  Should a party end the relationship, they would also be ending the contract, or promise to marry.  If the man ends the engagement, then the women can keep the ring.  Should the women end the engagement, the man would have the engagement ring returned to him.
The decision of Jacobs v. Davis, [1917] 2 K.B. 532 at p. 533 states:
“Though the origin of the engagement ring has been forgotten, it still retains its character of a pledge or something to bind the bargain or contract to marry, and it is given on the understanding that a party who breaks the contract must return it. Whether the ring is a pledge or a conditional gift, the result is the same. The engagement ring given by the plaintiff to the defendant was given upon the implied condition that it should be returned if the defendant” (i.e., the lady) “broke off the engagement. She did break the contract, and therefore must return the ring.” It seems reasonably clear that Shearman J. impliedly held that if the plaintiff himself had broken off the promise he could not get back the ring.”
 He further states at pp. 547-8:
“This I hold to be the correct legal view. If a woman who has received a ring refuses to fulfil the conditions of the gift she must return it. So, on the other hand, I think that if the man has, without a recognized legal justification, refused to carry out his promise of marriage, he cannot demand the return of the engagement ring. It matters not in law that the repudiation of the promise may turn out to the ultimate advantage of both parties. A judge must apply the existing law as to the limits of justification for breach.”
On a different topic all together, in employment there is the law of constructive dismissal.  The decision of Farber v. Royal Trust Co., [1997] 1 SCR 846 at p. 24 sums it up nicely:
“Where an employer decides unilaterally to make substantial changes to the essential terms of an employee’s contract of employment and the employee does not agree to the changes and leaves his or her job, the employee has not resigned, but has been dismissed.  Since the employer has not formally dismissed the employee, this is referred to as “constructive dismissal”.  By unilaterally seeking to make substantial changes to the essential terms of the employment contract, the employer is ceasing to meet its obligations and is therefore terminating the contract.  The employee can then treat the contract as resiliated for breach and can leave.  In such circumstances, the employee is entitled to compensation in lieu of notice and, where appropriate, damages.”
So here is my question to you, what happens when the man breaks off the engagement and wants the ring back stating that the foundation of the relationship, which is why the engagement contract existed in the first place, had changed so much that he had no option but to leave; she had forced him to leave?  I call this defence, Constructive Break-up and yes, I believe he should get his ring back, but please make sure the ring is worth more than the legal fees to argue this!
Food for thought
Suzanne