You've misunderstood GP - the $40 is not being taken from the hypothetical poor person - it's a 'get home in time for date night' product being sold for $40, so not affording date night means not doing it; not spending the $40.
The ability to afford food for the week is then unaffected, as it would be if the figure were $4 or $400.
However, this does impact gas, which could be significant if the difference in time is large enough.
(This is assuming that the poor person is not driving e.g. a (more expensive, fewer available as used) hybrid car that can idle in traffic without wasting gas)
The ability to afford food for the week is then unaffected, as it would be if the figure were $4 or $400.