I got spooked by interweb chat about the unreliability of the M9, so much so that I bought a second one for backup a year after my first. On one trip I brought both of them along, plus four lenses and I can tell you the weight was overbearing. I haven't done it again.
My first-bought M9 has never broken down on me, in 9 years of use, apart from Rusty Sensor Syndrome, which wasn't a breakdown as such. My other M9 I hardly ever use so I can't comment on its reliability or otherwise. It did need a sensor change for corrosion though, even though I kept/keep it in a dry cabinet.
My feeling is that KTM's M240 is not going to fail him on his trip. They are known to be as reliable as anything else out there. Taking it and the GR III will mean two sets of chargers and batteries and possibly two sets of SD cards.
I wouldn't worry about a backup camera as such, but these two cameras do complement each other, especially if KTM takes a 50 for his M240 rather than a 35.
It is also the case that the GR III on its own may well be the right choice - in 35mm crop mode its resolution is pretty much the same as the GR II resolution at 28mm.