I am curently trying to do the Children of Ur side quest. I am sending letters to the characters, but they are not replying back. I was just wondering if I have to be playing the game for them to reply.

Also, a question I have. When I add someones friend code, than try to send them a letter, their friend code is grayed out, thus I can not click on it. Any help with be appriciated. Thank you in advance for your answers and help.