Which is the correct term haploid daughter cells or haploid parent cells?
"Parent" and "daughter" terminology is indeed relative. Any parent cell has been a daughter at some point. However, in the case of the zygote there is no "parent" strictly speaking since it is the result of a fusion between two cells, that you may call "daughter cells" only if you are referring to the previous meiosis event.

In this particular case, there is no cell division, so no "daughter" is formed, so no point in calling the haploid cells from M2 "parents" either.