I think that's the point. Her helping him even though he hurt her so much just shows how strong she's gotten. Yes, she wouldn't do that for just anyone. Same situation, she wouldn't do it for Kumji, and she wouldn't do it for Hiyou. Neither one of those enemies hurt her as terribly as Soo Won did. and yet she does it for him, because unlike those two, along with the countless others she's fought against, Soo Won is good for Kouka. As much as I hate him, and as much as she probably hates him, he is a good king. He takes care of her people. He wants to protect them. So he is useful to her for now.
I don't think it's their old friendship, I think it's the fact that this person who protects and cares for the country she loves is in pain, and she will always help someone like that.
I like the way your brain works and I agree with the first part but i disagree on the last part. I genuinely think its bc of their past that shes like this. that deep emotional trauma is such a realistic aspect of her character and I've seen the same situation happen so many times first hand. also, off-topic but I like how deep you delve into the subject, you make for good conversations :)

oh no hes sick !1!1!1!1!1!1!1!
I hope it hurts :)
honestly, I hated him so much then I was indifferent and now I'm just annoyed with him. like the audacity. he has no right to act as familiar with them as he does and I genuinely hope he feels like shit.
also, idk why she's going so far to help despite the ptsd she's getting.
sis, there's no redemption in terms of morality for him. getting reintroduced is literally gonna open up old wounds and cause such a great amount of emotional distress. she needs to move on from him in every sense.