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The live from day to day and suddenly see an opening to improve their situation. I do not think they actually worry about what will happen when the friend returns.

And everything goes wrong before the friend returns so why should the film even address this?

The poor family failed with their pizza box scheme so they were destined to fail with this scheme as well.

the film pokes at both families.

One for being rich without too much merit ... the other being poor on their own faults. One of their faults being to want to make a quick grab without regard to the future.

No plot hole. The friend gets the rock he gave as a gift back on his head in Parasite 2.