I'd pull the memory andsee if its still crashing. Since it fit in there, the memory type is basically in the same family so it should have the same pinouts and thus not cause a major problem, I'm not sure,so if you pull and run the Apple Hardware Test CD you can be sure.
In one sense I'm with your gf's mom, why take chance when it was their screw up. Overall if it passes the AHT then you should be ok.
Mike