The couple conceived two children prior to their engagement. The donor of the ring participated in another relationship, the donee of the ring found out and ended their relationship/engagement. The donor wants the ring back. Is he entitled? The ring was not given as a gift, but upon the condition of marriage.
Divorce / Separation Lawyer
This question was answered excellently by the third response at the following link: http://www.avvo.com/legal-answers/if-a-couple-breaks-up-before-wedding-who-is-the-ri-6971.html .
Generally, unless there's some real clear facts indicating that the ring was given upon the condition of marriage, bad behavior by the donor would make it unlikely that a court would order the donee to give the ring back.
Real Estate Attorney
As far as whether the cheating you reference is sufficient bad behavior to alter the donee's entitlement to the return of the ring, that's a fact dependent issue. I don't think that's a question you can get an answer to on-line.
Talk to an attorney.