Montag lives in a world where anyone who is seen reading books, or appearing to be discontented with the government, or who is even a little bit odd, is often reported to the authorities, and taken away, imprisoned, or worse. And here is Montag, in a room full of fully brainwashed women, quoting poetry and yelling at them for being so shallow and empty. That behavior is completely notorious, and is likely to get Montag reported to the authorities. Faber is trying to talk some sense into Montag, and to get him to stop behaving so conspicuously, for his own safety.
Montag is in front of all of his wife's friends, insulting them and jabbing at their intelligence, and he has a book in his hand. Faber knows that this is close to a death sentence. He not only implores Montag to stop, but to
"play it as a joke, cover up, pretend you aren't mad at all. Then--walk to your wall incinerator and throw the book in!"
He feels that if Montag can play it all off as a big joke and then burn the book, he might be able to keep the ladies from reporting him later on. Faber is worried about Montag's safety, and his own. If Montag is reported, they could get to Faber too, and their plan to take out the firemen will be thwarted. Faber wants to play it safe in order to enact their plan, and to live. I hope those thoughts helped; good luck!
he was told by Faber because the ladies might report him