Chat the fake account and fool him that you wanted to buy a nude photo of her, and send him/her a link where to "contact" you.
The link is a bait to know here IP.
There is also this said law, try to ask authorities for help
Through a court order. In late 2016, a new law went into effect that sent chills down the spines of millions of people. Revisions to a U.S. federal security measure called Rule 41 gave the FBI and others more leeway while investigating online activity. The request for subpoenas to get IP addresses (and home addresses) is much easier now.
taken here :
https://whatismyipaddress.com/get-ip