I think you've already hit on the answer with the hydraulic lifters (tappets). If the oil pressure gets low enough in the engine, the lifters will bottom out and will not allow the engine to suck in the proper amount of air/fuel, nor open enough to allow enough exhaust out. This is a very bad situation.
Don't think the girls boyfriend got away scott free, though ... more than likely if they were down enough oil to cause this to happen, they've also introduced a significant amount of wear into the engine at the bearings and other wear points. Who knows how much added wear there ended up being, but you can be assured there was some.
One of the bigger things to figure out here is why was the engine so low on oil? It may have a leak or this may be the first time the oil has been checked in a very long time. You may want to enquire about his maintenance habits with the vehicle and get on him a bit if he's slacking on this ... you don't need your daughter getting stranded because her knucklehead boyfriend cannot even keep up on the very basic maintenance. Maybe this is a chance for you to get to know him a little better, eh? (Sorry, off on a tangent ... this isn't Parenting.SE, lol!)