Pretty sure she was also scared at first that Tom may have killed her, and didn’t want to rat him out for fear of her life. I think halfway through the interrogation she realized Tom may be trying to help her, but also sees him grab the vase that he was going to kill either her or Pritchard with, so she clammed up.

he removes the flowers from the vase. Without giving anything away, she sees that he is preparing retaliate if her testimony effects him in any way
You know, in DECADES of seeing this movie, it NEVER occurred to me that he was, in that moment, not only preparing to brain Pritchard but telling HER that he was preparing to do so.
It's kinda obvious now that you point it out.

If I may offer a different reading of the scene:
Keeping in mind the final information revealed about Costner's character, notice how tense the actress in this scene appears (or tries to) as he removes the flowers from the vase. Without giving anything away, she sees that he is preparing retaliate if her testimony effects him in any way. Pritchard (Will Patton) appears to maintain control of the interrogation in this scene, but visually the dynamic remains locked between Nina (Iman) and Farrell (Costner).
She knew who he was all along, and that's why he said thank you to her after she told everything to Pritchard.

Because if she hadn't, the movie would be over.

Because Kevin Costner was an A-List actor at the time, and he may have helped her get some acting jobs.

they threatened her with deportation,deportment is what they teach you at Swiss finishing school?

I think the film just wants us to accept her loyalty to Susan, and her trust in Susan's love for Tom. There was no hint whatsoever of her being a Russian agent. I think that would have been mentioned in the final scene.
BTW - that was Iman (aka Mrs. David Bowie) playing the part of Nina.
end

I didn't get the sense that she was an agent; just that she realized Farrell was in a bind having to conceal his identity as the lover while also attached to the case and was willing to stall for him. She also saw Farrell was about to bash him in the head with a flower pot and take the heat himself until Pritchard suddenly backed down.

I found it pretty unrealistic that she did not rat him out as she was given a serious threat and for her, Tom was no one. But following the final scene, it's pretty clear that she could also be an Russian agent assign to spy on Susan.

She was Susan's friend and was (I assume) happy that she was so in love. I think she protected him because she didn't think he killed her. When Pritchard backed off, she was safe. If he continued the deportation threat, she could have given him up but realized she didn't have to.