Your friend's bike likely has a high performance cam and a larger head gasket at the minimum if it has the HRC loom/ECU. He may experience ignition timing issues if he removes the HRC ECU.
According to OP they are just replacing the loom which shouldn't be a problem. I run HRC ecu and OEM harness on my bikes.
Isn't he going to have problems with turn signals, radiator fan, and guages by attaching a HRC ECU to a stock loom. The HRC unit isn't setup for operation of those ancillary things a steetbike has, is it?
I understand people do it with trackbikes, but didn't think it would work on a street setup.
The guages work fine, but you have a point on the signals, etc. In fact the HRC ECU is programable to accept the signal switch as a pit lane limiter so it may not work for real signals. I'd be interested in hearing if they can make it work. I don't have any experience with HRC ECU in a street application for that area.