They had to think about it long before there were "leftovers". I've been through ivf, not lucky enough to have "leftovers" because my situation was pretty dire to start with. However, before we started the whole thing, we signed about a dozen different releases, including a several-page notarized document about what to do with any extra embryos. Our choices were freezing, discarding or donating to science. The form was very thorough, including what happens if one partner dies, if we're divorced, if someone remarries, etc. So I assure you, these people weren't surprised with sudden leftover embryos.