People, stop talking non-sense.

The newer ARES G36s/Umarex G36s use a large barrel nut very similar to the real steal.

That would probably make them compatible with a WE G36.
I have not tried however.

But you can get a regular AEG G36E conversion kit and install it on your WE G36c. The "TM standard" G36s were G36c with a barrel extension screwed in place of the stock flash hider. Using a kit made for this on the tip of your WE G36c would work the same.
The only difference you would get from a genuine WE G36 is that the fake gas block is further forward on the carbine and rifle versions (G36 and G36K). And that can be remedied with a longer pin to install the gas block further down the barrel extension.

That said, unless the outerbarrels from the NEWER ARES G36s fit the WE G36c, you will have to source an "old school" extension kit. G&P used to make a very nice one. STAR/ARES had one pre-UMAREX, now they are quite hard to find.

....but then you'd have an ARES

Easier for him to just extend his current one, and buy a new GBBR :D