During chapter
8 Silberspiegel Refuge, 1937 part 1, talk to Leni on the Refuge's terrace. When talking to the Junta in chapter 10 Silberspiegel Refuge, 1937 part 2 report on Leni. In chapter
24 Silberspiegel Refuge, 1944 when talking to Leni, she will ask why you betrayed her.
The Betrayal achievement and the
Loyalty trophy contradict each other. If you want to get all the achievements in the game, you will have to go through the game again to
Chapter 10 Silberspiegel Shelter 1937 Part 2 and choose whether you want to betray Leni or keep it a secret.