Let's look at the question of "Christian liberty" from a purely physical perspective. Say that you go with your friend to pick up his child at a day care center. (Why the child is in day care rather than at home is another issue, but for the sake of argument, let's just say it's this way.)
As you go in with your friend, you notice the strong smell of feces as children play on a filthy carpet. Exposed electrical wires protrude from a wall not far from the corner where you spy rodent droppings. Asbestos insulation lies fallen under exposed heat pipes within the reach of children, and you suspect that the old paint flaking from woodwork may be lead-based.
Your friend seems oblivious to the dangers. So, do you warn him, or do you keep your mouth shut rather than say something potentially harmful to his Christian liberty as a parent?
It seems to me that a parent who loved his child and simply did not know of the dangers would want you to warn him. A parent who disregards or takes offense at your warning either does not love his child or lives in a state of denial.
Now there is a question....