If Tianna is at a battlefield, the hold on the other battlefield doesn't occur. Both players are on 7 points, Tianna is killed and they conquer the 2nd battlefield, do they get the last point?

Yes, you win the game.

According to Rule 447, you may only Score once per battlefield per turn. When you attempt to Hold a battlefield at the start of your turn, you initiate the Scoring process. Even if Tianna Crownguard's ability prevents you from gaining the point, the act of Scoring still occurs, and that battlefield is marked as "Scored" for the turn.

Sequence of Events:

1. Beginning Phase (Hold Attempt): You attempt to Hold the battlefield where Tianna is present. Because of her ability, you do not gain a point, but the battlefield is now marked as "Scored" for your turn (Rule 447).
2. Showdown/Combat: You kill Tianna Crownguard. Her passive ability ("While I'm at a battlefield, opponents can't gain points") is no longer active.
3. Conquer: You conquer the second battlefield. Since you have not yet Scored this battlefield this turn, this counts as a successful Conquer (Rule 446.1).
4. Final Point Check: To win the Final Point via Conquer, you must have Scored every battlefield through either method (Hold or Conquer) this turn (Rule 448.1.b.2).
   • You Scored the first battlefield via the Hold attempt (even though no point was gained).
   • You Scored the second battlefield via the Conquer.
   • Because you have Scored both battlefields this turn, you satisfy the requirement for the Final Point and win the game immediately.

As clarified in FAQ 8487 and FAQ 8452, Tianna only prevents the point gain, not the act of Scoring itself. Since the Scoring process was initiated at both battlefields, you have fulfilled the requirement to win the game.