Q. I help my non-religious elderly neighbors by doing shopping for them. When requested can I buy raspberries for them? If I instruct them how to wash them properly and inspect them, can I trust them that they will follow through, or should I better just tell them that I could not find any?

A. The prohibition of “Lifney Iver” or “Placing a stumbling block before the blind” is interpreted by our sages as: facilitating or helping others commit a Torah violation. (Talmud Avoda Zarah 6b) This prohibition is codified in Shulchan Aruch in several places such as Y.D. 148:5 pertaining the assisting of a Gentile in committing idolatry and in Y.D. 240:20 in regards to a father’s proscription of physically chastising older children, as this will only entice them to hit back, resulting in a capital offense. Tosafos, Ramban and Ran (ibid.) and other Poiskim, are of the opinion that this prohibition is limited to cases where the recipient is unable to transgress without the giver’s assistance. If, however, the idolater could reach the item independently, then one may hand it to him despite the certainty of the ensuing violation.
However as Tosafos (Shabbos 3a) and Shach (Y.D.151:6) mention, there is a discrete rabbinical prohibition against helping one violate Mitzvos, called “mesayea lidvar aveiro” (“helping someone commit a sin”). Igrois Moishe (Y.D. 1,72) deals with the question of mesayea when someone would perform the transgression nonetheless. (See question 207 in this forum)
Horav Shlomo Miller’s Shlit”a opinion is that in this particular case it is worth doing an act of kindness by washing the raspberries properly for them, while explaining to your neighbors the importance of cleanliness, insect non-contamination and keeping kosher. An act of altruistic graciousness will go a long way in achieving effective kiruv.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a