Iris didn't want to talk to her alone not because she was afraid Nala would do something horrible to her, since she used to have this kind of intention whenever she was close to her, but because Iris didn't think there was something for them to talk about.
She knew Nala was not in a good situation, since she lost her father during the attack and now she was so far away from her home, from the people she knew and was surrounded by pack members who didn't care much about her.
The King was gone and no matter how horrible it sounded, without the King, she was nothing. The respect she got was only due to the fact that she was some sort of favored woman to the King, though she couldn't be called his mistress.
Iris sympathized with her predicament, but she didn't want to have anything to do with her. She had stayed invisible until now, thus she wanted her to stay that way. She couldn't care less about how she was going to survive in this pack.