Without oil improvments and additives as the years go on(i.e. from the days of straight mineral oil), the engine would have sludged up before it reached 175k miles.
Engines fail for a number of different reasons, many because of valve train wear, loss of compression due to cylinder/piston(or piston ring)wear, damage due to overheating, simple neglect, crankcase contamination, damage from a car crash.... the list goes on
What was the cause of the rod snapping? Was the engine knocking before this event occured? If so, the owner of the vehicle had fair warning and should have had it fixed because the result of not fixing it is very clear.
Generally, engines fail because the parts inside wear out due to friction common to an engine. The better performing the oil the better control of the wear and longer theoretical life. Granted many people have other components wear out first, from suporting parts of the engine such as the fuel pump, computer components, etc. or the transmission fails. Some cars will run great until the day that they rust so bad that the body literally can't support it's own weight(happened to two of my friends, one more than half of the floor rusted through and the other the supporting frame around the suspension rusted and caved in at 367k miles.
Just think about this though: If you thought that all engines were going to fail by having the rod snap like you say, then most people probably wouldn't care, but engines generally last longer than 175k miles these days(especially diesels), and as I feel bad that your buddies engine blew at a modest mileage, there are other ways that engines fail.