When nightlight asks you if theres anything wrong about her wanting multiple partners, say no, then go back and talk to moose, moose will ask you to give a wilted flower to nightlight, do that then talk to moose again, then take moose as a mate and wait two days, moose will move to inside the tent thing and will be sitting next to grasswhistle, talk to them and then go talk to nightlight again, you should get the achievement then ^^