The gemara says that לפני עור prohibits m'doraysa one to give wine to a nazir if he had no access to it by himself (תרי עברי דנהרא). Tosafos explains that this applies even if the wine would belong to the nazir. Tosafos continues that based on this it would be prohibited to give a Jew who became a mumar to avoda zara food which isn't kosher, even to pass them their own food, because he will obviously eat it. But, Tosafos qualifies that this would only be assur if they had no access to it by themselves. Tosafos at this point is not addressing the issur d'oraysa, rather paskening a din, and seems to hold that if they would have access by themselves it would be permitted to assist them in getting the issur more easily. The Ran adds that the condition of תרי עברי דנהרא which limits the issur to a case where they don't have access by themselves, is only on a d'oraysa level, but m'drabonon it would be assur anyway because one is obligated to stop fellow Jews from doing an issur so he certainly cannot assist in it. The Tiferes Shmuel on the Rosh understands that Tosafos and the Ran disagree about this point and sides with the Ran. He then points out that Tosafos and the Rosh in our sugya both seem to hold that one can give non-kosher food to a Jewish mumar l'avoda zara so long as he had access himself, yet both Tosafos and Rosh in Shabbos 3a concur with the idea that one is not allowed to assist another Jew in doing an issur?
To answer this question, the Shach (y.d. 151:6) writes that the issur d'rabonon of assisting another Jew in doing an issur (even when he has access himself) doesn't apply to a mumar. Therefore, in our sugya Tosafos holds that one can assist a mumar in doing an issur, but in Shabbos where the aveira is being done accidentally one cannot assist. The Dagul Me'rvava explains the distinction of the Shach. The fundamental distinction is not whether the perpetrator is a mumar, rather whether he is doing it intentionally. The term "mumar" applies to anyone who is intentionally violating the aveira. The prohibition against assisting in the doing of an issur only applies when the violator is violating accidentally, but when he made a decision to violate the issur AND has access without me, there is no prohibition for me to assist him in doing the issur. In other words, when the situation is תרי עברי דנהרא it is assur m'doraysa and applies whether he is violating intentionally or accidentally, but when he has access himself and the issur is only d'rabonon because one is not allowed to be מסייע ידי עוברי עבירה it only applies when it is done accidentally but not when he made a decision to violate an issur.
The difficulty with the approach of the dagul m'rvava is that the source of the issur d'rabonon to assist someone in doing an issur is as the Ran writes - שהרי הוא מחוייב להפרישו מאיסור, והיאך יסייע ידי עוברי עבירה. The obligation to stop another Jew from doing an issur should make it assur to assist him in doing an issur even if he is doing it intentionally. Just as one has an obligation to prevent another Jew from doing an issur intentionally (הוכיח תוכיח), it should certainly be prohibited to assist him? The D"M seems to understand that the logic of "since you must stop him, you cannot help him", only applies when you have the control to be successful in stopping him such as one who is violating accidentally and will stop if you tell him that it is assur. But, when one has the ability to do it anyway and will do it anyway, the obligation to stop him doesn't apply because that isn't in your control, therefore there isn't any issur to assist him in violating.