Water pump failure is pretty common on 640's. When the water pump gives up,
the head overheats, blows head gasket ... and then cooling water can get into oil. Most seasoned 640 guys keep a VERY close eye on the tell tales and catch it early before any damage occurs. Many carry a spare water pump kit on board or parts to repair pump. Some carry a spare head gasket as well.
Once the water is in oil ... well, just a matter of time before all internal bearings are ruined. You said he rode for "days, refilling radiator several times a day". He should have stopped immediately and done repairs. After riding for days with water in oil ... I fear the result will not be good. Sorry to hear this.
I've been with KTM riding buddies on TWO occasions where this happened. The first time I towed the KTM to help. The 2nd time the owner ignored the steam and water leak. Motor destroyed after just two hours of riding.
Best of luck sourcing parts!