In Wisconsin one can be prosecuted for assisting suicide under ยง940.12 of the Wisconsin statutes, which reads: "Whoever with intent that another take his or her own life assists such person to commit suicide is guilty of a Class H felony." Under the facts as set forth in the question, knowledge that another has expressed suicidal ideation does not rise to the level of assisting such action so long as no affirmative act is done to encourage or assist in the suicide. If you know of someone who is talking about killing themself, by all means encourage that person to seek help. If you know of anyone assisting or planning to assist another in taking their life, urge that person to desist in any such conduct.