First off, it could be any number of things...
If it just stopped running, what was it doing before it stopped? Sputtering??
I know women don't always do the right thing, If she drove it while it was overheating, or while it was dangerously in the red zone, it could have blown the head gasket and/or warped the head...
I used to have cooling probs years back, and although my temps would raise I would ALWAYS be careful not to drive when the needle is in the red, this is very important.

Best case scenario is u/fiancee got spark plugs wet or ignition wet when filling coolant, loose wire or something, maybe even blown ECU fuse (under hood, in fuse box), ignition coil, fuel pump, it could be MANY MANY things...
U need to trouble shoot a little bit, or take it to a mechanic...
Do u have a Haynes Manual? if not, why not?

Worse case is u blew head gasket and or warped the head because it was driven when overheating (aka steam bubbles in your coolant don't transfer heat very well, and ur pistons burn at like 1500 deg F., conduction through fluid is much more efficient, like 1000 times, cause it's much more dense than a gas like steam) although possible, it's not likely, unless u saw some WHITE smoke coming out of tailpipe b4 it broke...

Gimme some more info, I can only speculate right now

More important, what sounds does it make??
Can u hear compression when it turns over?? or does it make an "empty" metallic noise...
Do u hear fuel pump turn on when key is turned to 'ON" position, does it make a "hum noise for like 3 secs" before u turn on starter...

-Mike