Seriously now beonthelevel, it is up to you to always proclaim that truth, in season and out of season as we say. No matter what, or who you are talking to. It is then up to THEM if they accept that truth or not. You have done your job well when you speak the truth, we don’t ever need to make exceptions for anyone. Just speak the truth quietly and clearly.
Here is a good quote from Benjamin Franklin:
Sin is not hurtful because it is forbidden, but it is forbidden because it is hurtful.
Maybe some of your unChristian friends will like that and understand it.
I’m not trying to say Ben Franklin was an unbeliever, I truly have no idea, I was just looking up some famous quotes of pasts presidents once and found that - wasn’t he the one that chopped down the cherry tree and said “I cannot tell a lie”!?