Did she ask him to give the necklace back? I think you need to ask her to be clear about what she has and has not tried, and what she is willing tondo to help you. Discuss whether, as his parent, she would be willing to reimburse you out of pocket so that retrieving the necklace and returning it to where it was purchased becomes her problem rather than yours.

When your child makes a mess, you clean it up. It's all well and good helping you move on but you have friends and family to support you through that and you'll get over him in your own time at your own pace. It can't be rushed. She needs to understand that this might turn into a legal battle that she and his father are going to be paying for anyway, so she might as well settle out of court.