In the Caspian Sea, you will encounter slaves' faction - they are forced to work for the local Baron. These people are innocent, driven by fear. The key to making Damir join us is to rescue the slaves, sneak past them and stun. At the end of the chapter, you will have a few side missions where you have to rescue slaves.
The photo of Giul's mother is in the bunker you visit during your searches for a satellite map. Give Giul that photo. Thanks to these steps, you will get the good ending at the end of the chapter. Damir will join Artyom's company (the Damir trophy/achievement).