Why doesn't Proctor want Mary to go back to court?
In Act II, before Mary returns home from the court, John Proctor scolds his wife, Elizabeth, for allowing Mary to even go into town to the court that day. He says, "You heard me forbid her go to Salem anymore!" Elizabeth explains that Mary was simply too willful for her to stop, and she describes the way all the girls are treated in the town now that the court is in session and people are being convicted of and confessing to witchcraft. John and Elizabeth go on discussing what Abigail had told him two weeks earlier, that Betty's and Ruth's illnesses had nothing to do with witchcraft. It seems likely that this is one reason John has tried to keep Mary from the court. He knows that the proceedings are false, that the girls are making everything up. Mary works for him, and so he'd prefer that she have no part of this charade and that she remain at home, working as she is being paid to do.
The place you are looking for is Act II, Scene 2.
In that scene, Mary has just come back from spending all day in court. She is telling the Proctors about what has happened in court that day. Specifically, she tells them that Sarah Good has confessed to making a deal with Satan and that Goody Osburn will be executed.
Proctor tells Mary not to go back to court because he is appalled at the type of evidence that was used to convict Osburn. He is also shocked that the court would not realize that Osburn was just old and a bit crazy.
To find more about this, just go to Act II, Scene 2.