does yuki partly responsible for what happened to geto?
Yes and no. It was slightly irresponsible for her to talk to him that way knowing his mental state but she was speaking hypothetically. She even gives him a disclaimer. I don't think anyone would have expected geto to do the crazy shit he did. And her conversation wasn't the only thing that led to his downfall. So it's unfair to blame her for that imo but she was also not careful enough around him
Geto was already mind fucked. I personally think gojo is more responsible than yuki cuz of him gained even stronger than geto, it made geto felt that their friendship got further.