The way I see it you have two options:
1) Change the quest state in your save file. Honestly I don't know which quest it is, so my advice here is less helpful.
Go to /scripts/events.gd and search "tishadecision". This describes the steps of this quest. Then edit your save file to change the quest to before they leave (or to just before they come back). Your save file is in (%appdata%/Strive/Saves) and both .gd files and saves can be opened in notepad++ or another text editor.
2) Start a new game, and copy the character from one game to another
a) Open your save file. Search for the name of the slave. Copy the whole text of the character.
- starts with {"@path":
- ends with "xp":0},
b) start a new game. Play a turn or two. Save it. Open the save file, paste the old character.
- BE CAREFUL WHERE - there are sections in the save file. Easiest is to find a char you already have, and paste the character directly before or after to make sure it's in the right section.
Option 1 is the "better" option, as it takes less work in the long term