It always cracks me up how the poor sap riding the bike always gets the full blame for the engine blowing up. Thats why I don't ride other peoples bikes.
yup, I was thinking the same thing parts don't just fail because somebody else is riding them. I've seen many KX 500 rods break, put alchol or nitrous and you'd better be looking at them frequently. I have a motor that has the Stewart head and window, it's only used for hillclimbing so it probably see's 5 hrs a year, I'll guarantee it'll have a new rod every other year.
I'd say your buddy just happens to be at the wrong place at the wrong time, I mean what can he be doing that you or anyone else isn't doing? Honestly there is only so much that you can abuse these things, was he tapped in first forever? that could do some damage. No, it was time for a rod and it didn't get it, and as a result you now have two rods.
I've loaned my bikes to friends and they surely didn't ride them like I do, but they've never had a catastrophic failure, yet.
While it is unfortunate that it happened, I'm sure you will do things differently going forward. maybe you should consider a new rod as well as the new bearings and seals, that will afford you a piece of mind, rods aren't that much money when you look at what it's going to cost you, a little PM up front will save on the back end.