My youngest child had this problem in elementary school - her classmate/gf copied her. It really annoyed my daughter. I talked to the girl's mom about it and it helped, but still, my daughter dissociated herself from the gf.

I have heard stories of women whose female colleagues copy their wardrobe.

When you are adults, you cannot talk to the mom of the copycat.

So you need to distance yourself from her. You cannot control or curtail her copying activities, but you can spend far less time with her than you used to.