The Mishna speaks about a case where the Ba'al Habayis is holding something in his hand from which the Ani takes, pulls out, and places down. The Ani is chayev a chatas for carrying out, and the ba'al habayis is patur. The gemara explains that it is patur u'mutar, meaning that the ba'al habayis didn't do any violation at all. Tosafos (top of 3a) asks why don't we consider the ba'al habayis to be in violation of lifnei iver, by providing the Ani with the possibility to carry out? Tosafos explains that even if it were a situation where the Ani were capable of picking it up without the Ba'al habayis lifting it up for him to take, it should still qualify as מסייע ידי עוברי עבירה - assisting someone in doing an aveira which is an issur d'rabonon? Tosafos answers that we must be speaking about a goy in a way where there is no ma'aris ayin because the object being carried out belongs to the goy. The Rosh disagrees and says simply that the gemara just means that no issur shabbos was violated, but there was certainly another issur d'rabonon violated. R. Akiva Eiger points out that if an issur shabbos were violated even accidentally the person would assume a status of מחלל שבת בפרהסיא even for the violation of a d'rabonon, whereas if an issur of מסייע ידי עוברי עבירה were violated, they wouldn't have such a status.
Regarding the exact nature of the violation to be מסייע ידי עוברי עבירה is a discussion itself. The Rosh writes that since we find a discussion about whether one must stop a child from doing an issur, the implication is that one must certainly stop an adult. Since one is obligated to stop an adult from doing an issur, they are certainly not allowed to assist in the violation. However, the Maharatz Chiyus cites the Rambam in sefer hamitzvos who says that the source for the issur to assist someone in doing an issur is the mitzvah of tochacha. Perhaps a difference between the two approaches is whether מסייע ידי עוברי עבירה can be considered an issur d'oraysa. According to the Rambam that it's based on the obligation to rebuke it would be d'oraysa, but according to the Rosh it would seem to only be d'rabonon. Another possible distinction is whether it would apply to assisting someone in not doing a positive mitzvah that is incumbent upon them. If the source is tochacha, it would apply to positive mitzvos as well, but if the source is an obligation to prevent people from doing issurim, it may only apply to negative violations and not positive mitzvos.