These are questions that spring to mind -
1 - has she ever copied anything of yours before? Clothes, jobs, boyfriends...?
2 - Would you say that your ring was an acquired taste? If it’s mind-boggling beautiful, and it’s ridiculous that all rings aren’t now made this way, then that would be different. But if it’s a bit odd (not that this is a bad thing by the way), that would be yet another thing.
3 - How long have you known her? Has she a relatively normal history with boyfriends/friends? Would it be crazy to think she might be slightly obsessed with you, or maybe your boyfriend? Do you share friends, or are you and she not in a group?
If you don’t hang around in a group, you’ve known her since childhood, the ring is spectacular in its perfection, and she’s never really shown any weird interest in either you or DF, then I’d perhaps let it go.
I’d be most likely to say “I really don’t want you to, because it seems weird, but I can’t stop you”.
But in my head I’d be shouting “WHY WHY WHY??????”. Actually, ask her that.