Well if he actually tried harder and showed them the photo maybe one of them could remember him lol But that would be trouble with one of the sisters (the one who likes the yankee version of him)
Crazy theory: Mutsumi actually exists and she is a hikkikomori with her own hidden room and she is the childhood friend.
Crazier theory: the "girl" is actually the loli mom of the quituplets. XD