Mass Effect 2 includes some excellent loyalty missions, and the one for Tali is arguably the best. In her quest, she stands trial for allegedly sending active Geth to the Migrant Fleet. Before proceedings get underway, you accompany her to the Alarei, which is the ship that her father was serving on and where she sent the supposed active Geth parts.
While clearing the place out, you find evidence that clears her name but implicates Rael'Zorah (her father). Afterward, the trial continues, and while Tali asks you not to reveal the evidence against her dad, it's up to you how the hearing plays out. Here are all of the possible outcomes.
Let Tali Be Exiled
It's possible for Tali to be exiled from the Quarian Fleet and never allowed to return again. To get this outcome, you need to say "We Have No Evidence" when prompted. With nothing to defend Tali, the Admiralty Board will find her guilty and give her six hours to leave the fleet forever.
On the bright side, she will appreciate that you didn't snitch on her father, and you'll keep her loyalty. Yet, in ME3, she won't be on the Admiralty Board, which could lead to disastrous results for the Quarians or Geth.
Save Tali But Incriminate Her Father
If you want Tali to remain a member of the fleet, you can share the evidence that incriminates her father. By choosing "Yes. Here Is The Evidence," Shepard will play the recording of Rael'Zorah where he explains what he was doing and how his daughter had no part in it.
In response, the Admiralty Board posthumously exiles Tali's dad while clearing her of all charges. Naturally, Tali will be upset at your actions and you won't gain her loyalty. To make matters worse, the reveal of Rael'Zorah's misdeeds will cause friction within the Admiralty Board, according to Tali. However, keeping the Quarian a part of the fleet helps you broker peace between her people and the Geth in the next game.
Save Both Tali And Her Father's Reputation
It is possible to keep Rael'Zorah's reputation intact while stopping Tali from being exiled. As this is the best outcome for all parties, it takes a bit more work to achieve. Yet, there are two ways to do it.
Use Charm Or Intimidate
If you have the required morality points during the trial, you'll be able to pick a Charm ("I don't need evidence") or Intimidate ("you're wasting your time") dialog option. Shepard will then give an impassioned speech, possibly about the Board using Tali's trial for political reasons.
The Commander's words convince the Admiralty Board to drop all charges against Tali and her father. So, your Quarian friend remains a part of the fleet and becomes loyal to you.
Rally The Crowd
Another way to help Tali is to get the crowd on your side. However, "Rally the crowd" will only be an option if you do these things first:
- At the end of Freedom's Progress, allow Veetor to go with Tali.
- On Haestrom (during Dossier: Tali), convince Kal'Reegar to stand down instead of attacking the Geth Colossus.
- Speak to the Admirals before leaving to retake the Alarei (during this loyalty mission).
These actions will allow you to rally the crowd, which will persuade Veetor and Kal'Reegar to come forward and defend Tali. The others will then side with them, and the Board will clear Tali of all charges. As a result, she'll avoid exile and be loyal to you.