Likvod Harav Shlita,
At a business meeting, my friend was served a kosher turkey sandwich. Another businessman, who is Jewish but not observant, said to him, “I see you have a turkey sandwich – I haven’t had a good sandwich in ages; as soon as this meeting is over I am going to go to Subway (a non-kosher restaurant) and order one.” My friend then noticed that his sandwich was ripped and not properly double-wrapped. Would he have been allowed to offer the non-observant Jew his sandwich, so as to hopefully prevent him from eating real treif, or would this have been lifnei iver, as the sandwich is basar shenisalem min hayin, especially considering that there is no way to know for sure that the man was truly going to go eat at Subways?
Assuming what you know that the sandwich that was given was indeed was from a kosher restaurant, it would be the correct thing to give it to the friend to eat, which isn’t considered lifnei iver because you are saving him from doing a worse issur, even though you are not 100% sure that he will do it.