MIL harassing us over "marital asset"

My husband lost his father 4 months ago. He's been hurting a lot over it. His mom was and still is awful and he found out his mom had been cheating on his dad. What this is all over is this ring my husband's dad gave him when he was 16. When my FIL was 16 his dad gave him this ring that belonged to his mom, so my FIL passed it down to my husband when my husband was 16. My husband didn't really ever wear it because he found it "girly" but he's been wearing it since his dad died. I guess the ring is worth at least 5k and his mom is wanting it. She said because she was married to my husband's father the asset is still hers. His dad gave it to him so I am not sure how true that is. But she's saying she needs the money because she already blew through his dad's life insurance. She said it's legally hers and doesn't want to have to take us to court over it. My husband has blocked her and she's sent some of her friends to try to negotiate it. Does she have legal rights over something that my FIL got in his teen years and passed down?