After locating Fate Garrus will finally get to face Sidonis. The Renegade option is easy enough, have him kill Sidonis but there’s a second option. Here’s how to save him and gain Garrus loyalty.
If you get the Renegade option Emily Wong can be heard stating there’s no funeral since Sidonis had no family.
The Paragon option open up with Warn Sidonis. He’ll try to leave but trigger a Paragon option to keep him alive. Choose Keep Talking and Don’t Do it Garrus. Garrus will Shepard to tell Sidonis to leave.
In the aftermath you can hear a news report of Sidonis turning himself in. This will earn you Garrus’ loyalty.