The Gemara in Masechet Pesachim (Daf 22) discusses the prohibition of "Lifnei Iver Lo Titen Michshol" – "Do not place a stumbling block before a blind man" – which the Sages apply to the prohibition of assisting a person commit a sin. For example, this prohibition forbids handing a cup of wine to a Nazir (person who took a vow to abstain from wine). Even if the Nazir could access the cup independently, it is nevertheless forbidden to assist him by handing him the cup, (by force of Rabbinic enactment, as opposed to Torah law).
This prohibition also includes assisting a gentile in the violation of a law that applies to him. For example, it is forbidden to hand Eiver Min Ha'chai (meat taken from a live animal) to a gentile, since the prohibition against partaking of this meat applies to Jews and gentiles alike.
One particularly interesting application of this Halacha which the Gemara mentions is striking children. It is forbidden to strike one's older child, if doing so will likely cause the child to hit the parent, which constitutes a Torah violation. Since "Lifnei Iver" forbids doing something that might lead another to commit a sin, a parent should not strike an older child, even for purposes of Chinuch (education and training) for that older child may strike back.
The Ben Ish Chai (Rabbi Yosef Chaim of Baghdad, 1835-1909), in his work of responsa "Torah Lishma," raises the question of whether it is permissible to share with one's fellow some information that will likely arouse his anger. Would causing anger in this fashion fall under the category of "Lifnei Iver Lo Titen Michshol"? The Ben Ish Chai finds the answer in an incident recorded in the Gemara (Kiddushin 32), where Rav Huna once tore some expensive material in the presence of his son, Rabba, as a test to see if his son would grow angry. The Gemara asks why this did not violate "Lifnei Iver," given the possibility that Rabba would respond by acting disrespectfully to his father, in violation of the obligation to respect parents. The answer, the Gemara says, is that Rav Huna from the outset forewent on his honor, and therefore his son would not transgress this Mitzva of honoring parents by acting disrespectfully towards him. As the Ben Ish Chai observed, the Gemara appears not to have been concerned lest Rav Huna's actions would cause his son to grow angry. Apparently then, doing something that could potentially arouse anger would not transgress the violation of "Lifnei Iver," and it is therefore permissible to share information or discuss a topic with somebody even if this might arouse anger. Of course, this applies only if one has good reason to share this information, and does not do so for the specific purpose of causing him to grow angry.
Summary: The prohibition of "Lifnei Iver" forbids helping to facilitate a violation, such as handing a cup of wine to a Nazir or handing meat taken from a live animal to either a Jew or a gentile. Likewise, one should not strike an older child, since the child might then strike the parent, in violation of the Torah. One may, however, share information with a person even if he will likely grow angry as a result, if he has some constructive purpose in doing so.
