An IQ of 99 is considered within the normal range so this is not something that a judge can use to declare someone incompetent. Can you tell me what type of behaviors, symptoms do you see that worry you about this individual?
As much as your worry about her I am not sure that you can do anything about her behavior. I understand that she is making a big mistake and your can see that. Unfortunally, if she is over 18 you can't do anything legally. I think the only thing you can do is give her a bit of space to come back to you with her questions. Are you afraid that she will be manipulated by others? How old is she?
Hello there I can't even imagine how worrisome this can be for you, to have someone manipulating her this way. I think being supportive without being very involved is probably the most affective thing to do. You don't want her to worry about calling you back because she does not want to hear I told you so. She seams emotionally immature for 23 but she will learn her own lessons. Give her some space and then communicate with her. Is she your daughter? I think this is the hardest thing for a mother to do to see her child making a mistake that she can't prevent. I hope you are getting some support to cope with all this.