This is unlocked during the mission The Letter, located in the city of Dongguan. Begin the talking when you arrive, which will soon lead to your next recipe and the Turnip Cake achievement. You will be given a letter to translate, take it to your grandma for some talking and the green options Fear of Loss, Trust in Love, and Always a Tong.
Go Outside and then to the Roof to talk to the guy, using the Trust in Love option on him. Back to the living room to talk some more, then back up to the roof. Use the Always a Tong option this time, clicking through the following dialogue until The Letter achievement unlocks.