a/the liar is not believed when he tells the truth
A person who gains a reputation for being dishonest will not be believed about anything, even the truth. You've become so adept at taking advantage of your friends' goodwill, that you're going to run into trouble should you even genuinely need help. The liar is not believed when he tells the truth. You need to stop spinning these tall tales, Frankie, or people will start believing you to be nothing but a liar—and a liar is not believed even when he tells the truth.
liar is not believed (even) when he tells the truth
Prov. If people think that you are a liar, they will not believe anything you say. As it turned out, Fred was right when he warned his friends that the police were planning to raid their party; but they paid no attention to him, since they knew him to be a liar, and a liar is not believed even when he tells the truth.