Sounds to me like you've done a good job, but if there's no rust, there's no reason to oil fittings. If there was rust, then this would be M15 fittings with the paint gone, which is no big deal.
Personally, I never, ever oil, soap or treat the leather inside. This is for several reasons. The chemicals in these treatments are formulated for new leather, rather than that over 100 years old. If the treatment soaks trough to the lacquer, it can cause problems and some treatments will actually damage old leather.
Much of the time, I just dust off my new acquisitions and keep them in a place where I can monitor humidity. For example, I bought a Bavarian enlisted helmet with red-rot to the neck guard edges. I didn't try to fix it, but put it in a place where I know the humidity is good. This little area did nothing to detract from the overall appeal of an otherwise 100% nice helmet.
I think there are guys here doing this regarding your friend's storage techniques: ](*,) If I had a friend doing that, I'd give him a firm, 'preserve them or get rid ofl them' speech. Many people think that these artifacts belong to them, when they actually don't. If they belong to anybody, they belong to their long dead initial owners. We're just fortunate enough to have them and preserve them for future generations. We're well past the time of looking at these as something grandpa brought back, that the kids can play with or can be left to rot. To me, this is like a firearm collector who stores rare weapons in a musty basement and just ignores the rust starting to bite the metal.
So, keep up the preservation and maybe present your friend with a convincing argument to take care of those helmets that he's slowly killing.

Ron