The Gemara in Masechet Shabbat (4) establishes the principle known as "Ve'chi Omerim Lo Le'adam Hate Kede She'yizke Havercha," meaning, one may not commit a sin for the purpose of preventing somebody else from sinning. If a situation arises where preventing one's fellow from committing a transgression would necessitate performing a forbidden act, the forbidden act in question does not become permissible. One may not commit a transgression even where this is necessary to prevent his fellow from committing a transgression.
The Rishonim (Medieval scholars) question this principle in light of other passages in the Talmud which appear to indicate otherwise, that one may commit a minor transgression in order to prevent another from committing a more severe transgression. Tosefot resolve this discrepancy by distinguishing between intentional and unintentional violations. If a person knowingly prepares to commit a forbidden act, somebody else may not commit a transgression in order to prevent that person from sinning. Since he sins intentionally, others should not compromise their own observance to rescue him from sin. When, however, a person unknowingly commits a misdeed, others should make every attempt to prevent him from doing so, even if this entails committing a minor transgression.
Thus, in Masechet Shabbat the Gemara speaks of a case where a person sins intentionally, and it therefore prohibits committing a forbidden act to prevent him from sinning. In the other contexts, the Gemara speaks of an unintentional violation of a grave prohibition, and thus allows intervening even when this entails committing a minor transgression.
One interesting application of this rule appears in the Mishna Berura (commentary to the Shulhan Aruch by Rabbi Yisrael Kagan, 1839-1933) in the context of the laws of Tefila (90:84). Halacha strictly forbids praying the Amida in close proximity to excrement; doing so constitutes a Torah violation and renders all the Berachot one recites "Berachot Le'vatala" ("wasted" Berachot, which one is forbidden to recite). A question arose regarding a person who notices as he prays the Amida that his fellow several feet away is praying directly in front of excrement but does not notice (such as if the excrement does not happen to emit an odor). May this individual interrupt his Amida – which is normally forbidden by Halacha – in order to notify his friend that he cannot pray in his current location? The Mishna Berura applies Tosefot's principle to this case and thus allows the person to interrupt his Amida for this purpose. In this situation, a person commits a violation unknowingly, and therefore others should intervene even if this entails committing a minor violation. Hence, one should interrupt his Amida – which constitutes a minor Halachic infraction – for the purpose of saving his fellow from the grave Torah violation of praying the Amida in close proximity to excrement.
Of course, one must exercise great caution before attempting to apply this principle. People might be tempted to "abuse" this rule by enlisting it to justify forbidden practices that they in any event wish to perform. Each case must be carefully assessed and discussed with a competent Halachic authority, and one should not independently conclude upon the right to commit a forbidden act to prevent his friend from sinning.
Summary: In principle, if a person unknowingly engages in forbidden activity, others should intervene to stop him even if this entails violating a minor transgression. In practice, however, one should not intervene in such a case before consulting with a competent Halachic authority for guidance.
