Another lo sa’aseh that’s very, very easy to violate when one speaks or listens to lashon hara is “לפני עור לא תתן מכשול”, not to put a stumbling block in front of a blind person.
The Gemara tells us that if I loan you money with ribis, if I charge you interest, I violate a lo sa’aseh, I’m loaning you money. Additionally, you violate a lo sa’aseh because you’re not allowed to borrow with interest. But additionally, when you borrow from me, you’re over lifnei iver. I could not possibly loan money with interest if you didn’t accept it and the flip side as well, you could not possibly borrow if I didn’t loan it. So not only do both of us violate the prohibition of loaning or borrowing money with interest, each of us violate “לפני עור לא תתן מכשול”, in front of a blind man do not put a stumbling block as well. The definition of that lav is if you prepare the way for a Jew to sin. We have a unit, we are one body – I am responsible for you, you’re responsible for me. If I prepare the way for you to sin I violate a discrete lo sa’aseh called “לפני עור לא תתן מכשול”. I helped you to sin, I caused you to sin, and that’s for me a lo sa’aseh.
Explains the Chafetz Chaim, a person who speaks lashon hara is almost always going to violate not just the problem of “לא תלך רכיל בעמך”, of speaking lashon hara, additionally he’s causing people to listen to lashon hara, most likely to be mekabel lashon hara and he’s involved in this lo sa’aseh called “לפני עור לא תתן מכשול”. He’s putting a stumbling block, he’s making it easy for people to violate the issur of listening and being mekabel lashon hara.
Additionally, if you listen to lashon hara there are many times when you are also, not just in the problem of listening, you’re also involved in lifnei iver. Because if you and I are speaking alone I could not speak if you wouldn’t listen, so if you’re listening, you’re helping, you’re aiding and abetting me in speaking. The only distinction might be in a group if I were speaking to ten people and you would walk away it wouldn’t make a difference, so it’s not always true that the one who is listening is over lifnei iver. But certainly in a private conversation it’s true. And this lifnei iver is something that is often a problem in speaking lashon hara because I as a speaker am potentially over lifnei iver and you as a listener as well.