One question I'd ask is whether a head gasket failure typically begins with
a sudden Pop sound. If not, then maybe there's something else wrong with the
engine. I don't know the answer to this one, but before assuming the worst,
it's worth making sure the engine is really kaput.

I once had an 02 sensor pop out and that definitely makes the van sound like
an ancient lawn tractor. Could her problem be something with the exhaust,
maybe a sudden failure of the cat or muffler baffles? That might explain the
loss of power, maybe not failure to start, though.