Good for her.
Since he was married, the ring was not an "engagement ring", but a gift.
Legally, you can keep gifts.
I am unsure of the legal status for engagement rings...traditionally if you break an engagement you send it back. If he breaks it, you can keep it...
Since essentially he "broke" the engagement because he had no intention of marrying her, she should keep the ring.
Serves the SOB right.
Presenting an engagement ring is based on a promise to marry. If the date of marriage was after a divorce, which it must be, and the divorce is final except for 90 day wait, then the divorce has to be presumed certain and the promise to marry is still valid.
Most likely, this guy will win on appeal, if he can afford the lawyer's fees.