[6ix]

Unless you're part of the intelligence community, this is pretty much impossible.

Agree, even then (part of an Intelligence community), It will be very difficult to obtain the source IP.

Try SE'ing this person Into a chat gateway outside the Tor network, and use either of these links to obtain his/her IP:

http://blasze.com/
http://whatstheirip.com/

Good suggestion,

OP I really think SE is your only option here, try and convice your victim by talking to him, getting to know him etc..