Q. Is it permitted for someone that is careful to eat only products that contain Chalav Yisrael, to serve to his guest that eat non Chalav-Yisrael pastries and chocolates as long as they have proper Kasher supervision? Or since for him it is prohibited food, he may be transgressing on Lifne Iver? (placing a stumbling block before the blind).
A. Poskim disagree if it is permitted for members of different communities such as Ashkenazim and Sefaradim that may maintain different traditions on food prohibitions, such as the sirchos, (scar tissue and strand like adhesions), that develop on the lungs if they can feed or sell the meat prohibited to them to members of the community that permits them. Mabit (1: 21) and K’sav Sofer (Y. D. 77) permit, while Shaar Hamelech (H. Ishus 9: 16) prohibits. Similarly, Yalkut Yosef (Issur Veheter 2: p. 463) presents community disagreement in regards to cheeses made from milk of a particular treif animal. There are similar cases in regards to cooking on Shabbos (Amira Lenochri p. 140) and others. Many Poskim permit.
Horav Shlomo Miller’s Shlit’a opinion is that as long as the guest are aware that the food offered to them is not Cholov Yisroel and they usually consume it, there is no Lifnei Iver prohibition. The Rov Added that in this particular case there may be additional reasons to be lenient, since the milk is served in a mixture, and it may be only whey or other extracts of milk.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a