First you got to find out the IP address of that User.
The Next Step is to Trace that user using his IP address.
-a Displays all connections and listening ports.
-e Displays Ethernet statistics. This may be combined with the -s option.
-n Displays addresses and port numbers in numerical form.
-p proto Shows connections for the protocol specified by proto; proto may be TCP or UDP.
-s option to display per-protocol statistics, proto may be TCP, UDP, or IP.
-r Displays the routing table.
-s Displays per-protocol statistics. By default, statistics are shown for TCP, UDP and IP; the
-p option may be used to specify a subset of the default.