I understand diagnosing the problem can be difficult when it's someone else's car.
The best thing we can do to identify the problem is to look at exactly what happened before the car died, and identify any trouble codes saved by the car's computer.
If the car felt like it lost power but wasn't shaking/sputtering you may have a bad fuel filter or some other faulty fuel system component. You can cheat with a can of starting fluid (use sparingly) to test if the car will fire up. Remove air filter and spray directly into the vehicle's intake manifold.
If the car was shaking and sputtering I'd start by checking your ignition system. First look at the spark plugs, what's their condition, age, and proper gap? If those look ok, I'd inspect the wires for cracks or abrasion spots. The coil packs are the last thing I'd change, as they a very expensive.
Keep in mind these components may be bad even if they look ok.
If you can get me some more information about the OBDII diagnostic codes, we can help narrow down the problem further.
Hope is info helps,