I'm sure this topic has been debated before - if an engagement is off does he get the ring back? - but it's a bit more personal and complicated.
My SIL has been engaged for 6 years and she and her common law ex-fiance have 3 kids together - ages 5, 3 and 1. A couple weeks ago she caught him red-handed with her friend in bed downstairs in their own house. He's now out and has found a place of his own, but is refusing to contribute to this month's finances (and likely the future bills as well) because he needs to put down $ on his new apt. Whatever. I told DH that she should sell the ring and use that money to get buy for the next few months. You should see this ring, btw, a huge 1 ct. solitaire and 28 small diamonds and she could probably house/feed/pay bills for 3 or 4 months with the money.. I know that engagement rings are in many cases must be returned to the giver because it symbolizes a contract of sorts - the contemplation of marriage - and that the ring is the 'consideration' - the monetary thing given to make it a legal contract.
In this situation, legally or morally, do you think she is obligated to return the ring to him? I am not sure if adultery entitles her to keep it, but it might. IDK.