when to keep your big trap shut?

So last night Liz and I went to my friend Kimski’s house to hang out with her and her coz from Chicago. So we sat around eating pizza, sour patch kids, and peanut butter brownies. I was a little dismayed by the behavior of some of the people there. Everyone was a “Christian” except the coz Rena. Friday night Rena Liz, and I went out to dinner, than ice skating. I thought it was great, we had an awesome time of fun food, and I felt like at the end of the night, we were friends.
Saturday night was the pizza feed, everyone was sitting around the living room munching on the pizza and having fun. I do not think that anyone was trying to but the group came across as “Christian” and it was not in a fun and pleasant way. Rena said something like oh crap, and than felt bad and had to check to see if that was OK for her to say, as if by saying crap she might offend some of these “Christians”. I just felt like that was so sad, that people have to change the way they act around Christians. To an extent I can understand and applaud the effort if you can bring around change in someone else’s attitude through witnessing to them through your actions, but why should we as Christians shame people into a place where they feel convicted by anything that comes out of there mouth. If they have to adopt a game face just to be around us I say that we are presenting a wrong image.
Later that night, I was taking L home, we started to talk a little about that, L was feeling the same way. She felt like a lot of the attitude was centered on one person, X. X emanated an aura of “correctness” X felt that they needed to correct other peoples speech, Liz slipped out the big sh**, X corrected her. It offended Liz that someone whom she had never met before, felt like it was their place to correct her. In all honesty it could have been done with all the love in the world, but it did not come across that way.
I am just saddened by even the possibility that we reinforced any negative preconceived notions that Rena had about Christians. Where is the line? What do we demand from people and where is there room for grace?