I worked for Cossor/Raytheon and have worked on all types of kit incl IFF for Heli's.
The problem you may find is that your friend wants to keep everything original, sadly the kit is only designed for 25 years of working life, but are often supported after this time when 2nd users like far east military users take them on - in truth if it is not required like IFF in a warzone - they leave it switched off or fit a modern avionics suite!
If you want to know how owners want to 'keep original' someone asked me to build the original IFF for a Spitfire! I had all the information to do it including the original prints, schematics etc but when i quoted him £270k to build a new one from scratch, he decided he wanted to keep the original box so i suggested to him to have all the internals all modern adapted from an of the shelf transponder. It worked and was far cheaper.
My advice, if the kit is that un-reliable - buy a modern transponder off the shelf, gut out the old case and get a good electronics house to wire it in for you, i know that sounds easy but there is alot of work required.
You should be able to find a suitable interface with the right wiring schedules for the aircraft, if anything gutting it out would save a few lbs in weight.
and no im not volunteering to do it! =)