Engine cracked water was boiling?

Hello. I borrowed a car from a friend , i went to another city with it all was ok , we were coming back home all was ok till at one point i was on motorway and while overtaking another car i heard a noise while i was accelerating and after that the engine just stopped and saw like smoke coming out. I pulled over not that i had any other choice , when we open we saw the water boiling u could definitely boiled eggs for breakfast. As a notice the car is pretty old i think more or less 94 reg or something. i did not really cared to check stuff before i used it thinking that a friend wouldn`t borrow u a car knowing is not right to drive it. Ok now the problem is , he`s telling me that he needs another engine. Any ideas what could of happen? i mean an engine doesn`t just stop after dunno 50 miles of driving and the water wouldn`t boil with no reason.Another problem might be , my friend got off the car and ofc with no experience in cars she added cold water over the boiling water which at that point i wasn`t thinking either because is the first time it happen to me it is possible that she might of crack the engine this way? i mean she added water over the boiling one , we waited after that a while and then i tried to start the engine but it would not start. Please any ideas would be helpfull. thank you