Jeez!! Lucky he walked away from that one!!
Just going by what we could see on the vid, it sorta looks as though a few of the wheel studs on that LH rear broke, severely stressing the other studs holding that rim onto the axle, & ultimately causing that rear wheel to part company with the rest of the vehicle at speed & then the roll-over...
For my 2 bob's worth, I'd guess that the list of 'possible causes' might include excessive loading &/or travelling 'too fast' on & for those roads; or maybe even just the massive pounding the vehicle & every component on it was subjected to by all the corrugations on the tracks up that way - it doesn't really matter what
speed you travel at, it's all pounding & it can shake things loose &/or break them!! Regular (at least daily) checking of the wheel lug nuts is a good habit to get into because of that - but you just can't re-tighten them or tighten them a bit more every time, you hafta be careful not to OVER-tighten wheel studs, especially on those sorts of tracks & travelling at 'speed' (even if it's only 60-80 kph) Too much tension/torque loading on the wheel studs stresses the metal to start with, then if that is combined with hammering the studs by driving over about a million corrugations, it can significantly fatigue the metal of the studs & lead to breakages & failure - & once 1 goes, if it's not picked up immediately, the others follow quickly!! It's a fairly common thing to see up that way.
All up, it's just another reason for getting those 'professional' pre AND post-trip inspections - even if you are reasonably sure it's all fine, it's best to make sure it all looks good before you go & fix anything iffy well beforehand; then once you get back, get another set of eyes to check it all over & make sure nothing is (obviously) damaged or dangerous...
As for getting that rim fixed, while it might be feasible & maybe even possible, personally, I wouldn't ever consider using a rim damaged like that again AT ALL!! (Except maybe as a wall mounted 'memento' of such a lucky escape!) The stress loading placed on it before & during the 'accident' may well have damaged the very structure of the metal in the rim, potentially making it far more likely to fail catastrophically at the lightest of future knocks!! The rim has visible circumferential grooving from the disc as well as significant damage to the hub section that supports the wheel studs & holds the rim onto the vehicle, either of which may have introduced undetectable weak points into the metal of the rim - and if either of those weak areas fail down track somewhere, the tyre will likely either blow right off the rim or let the tyre part company with the vehicle at speed.... And he's just seen first hand what loosing a wheel at speed can do & was extremely
lucky to walk away from the resulting accident!! Why even chance
that happening again?! He might not be so lucky next time!!