Just went to have a closer look at the cups and there was the rogue bearing, sat in the non drive side cup, happy as you like.
Anyway, yes, I'm not tempted to put the cones back into service, and I think I'll probably choose my battles and try and get them to buy a new chain for the bike before anything else. Let's hope there's an equivalent wheel knocking around that we can swap in.
Thanks for the tip about the dust cover upgrade, Brucey. Looking at the state of the drive side cup, it would probably warrant a whole new freehub body, as well as replacement cones and bearings. Cheap compared to the cost of a new wheel, but expensive in terms of what the charity usually spends on parts.
The French Tandem's suggestion of polishing compound seems very gentle and reasonable after finding a blog post last night where some people were describing the use of an angle grinder to smooth out pitted cones!
I've not seen the tapered end of a cone fractured off like this one before - that's about 2mm gone off the end and there's another section similar to that, maybe 50% of the circumference in total. Is that a typical failure mode? I've tried googling to see if others have posted similar, but it's one of those where I'm not really sure what words to use to describe it.
Could it have been so badly assembled that the cup/cone were not in contact with the bearings?
I couldn't possible comment. (But a volunteer-powered effort with varying skill levels and negligible quality control...)
If the bearing gets slack enough the spindle rubs on the inside of the hollow bolt that secures the freehub; I think that is what has happened here
Good call - I took the freehub body off and the worn area on the axle threads is an exact match for the length of the bolt, which is worn on the splines.
I guess if the cones weren't tightened in on the bearings enough that might have allowed for the bearings to sit down on the thinner end of the cone where it wasn't strong enough to take the force. The bike would have been heavily loaded with it's most recent user, as well.
Both the headset and bottom bracket are loose - looking forward to more learning opportunities there!