Tosfos says in Shabbos Daf 3a that even when there isn't a problem of lifnei iver since the person would be able to get to the prohibited thing in another way, still there is an "issur d'rabbanan" (rabbinical prohibition) to give over the "prohibited thing" since one is obligated to be "mafrish" (separate) the the other person from issur.
However we know the idea of "כל ישראל ערבים זה בזה" that every single Jew is connected to another and responsible for their spiritual livelihood as well. This we know from the pasuk, "וכשלו איש באחיו" (See the pasuk here (Viyikra 26:37) and Gemorah Shavuos Daf 39a).
As well we have the mitzvah of "הוכח תוכיח" (See pasuk here (Viyikra 19:17). That a person must rebuke his fellow.
If so why did Tosfos simply say that there would be an "issur m'drabbanan" to give over a prohibited thing when there is no problem of lifnei iver. Seemingly because of the idea of "כל ישראל ערבים זה בזה" and "הוכח תוכיח" which are from the Torah (m'doraisa) a person shouldn't give over a prohibited object even when there is no problem of lifnei iver. Why did Tosfos not choose to use this reasoning (which as I pointed out are from the Torah.)