Miss Manners: I helped him pick a ring, but feel betrayed by his choice

A woman shared her dream engagement ring design from a prominent jewelry brand with a friend, believing she was secure because the friend had previously expressed dislike for it. However, the friend is now purchasing that exact ring for someone else. The woman feels betrayed by this choice, as she is currently single and unable to afford the jewelry herself, despite having helped the friend with the proposal process and ring selection.