Dishonesty - someone is always hurt by it: the person who is doing it, because they can't reach the truth without beginning with the truth. "This is now causing them a lot of mental Dukkha." Because the Law of Kamma is the Law of Kamma, whether we like it or not. "They ask for help from a friend but this person is concerned about compromising their precepts particularly if their advice is not followed. Please comment."
Give honest advice. You don't have to compromise your principles just to be friends with somebody. I would never do that. Perhaps they need you to give some honest advice to help them change. Nobody's friendship is that important, to make us actually compromise our principles.
Remember, we are the owner of our own Kamma, heir to our Kamma, born of our Kamma, related to our Kamma, abide supported by our Kamma and whatever Kamma we shall do of that we will be the heir.
So be very careful that the advice you give is not based in the eight worldly conditions of wanting praise instead of blame, gain instead of loss, etc. We might lose our friend but that's better than losing our principles. If we lose our principles, if we compromise too much then later we will have self-hatred and regret.
Be careful and give honest advice. But they don't have to compromise their principles either, they can make an honest decision. Whether they follow the decision, that is up to them. Whether they follow that advice is also up to them. We see what belongs to them and we see what belongs to ourselves.
Some people, when they come to us for advice, often don't like our advice. But we do it with the compassionate intention and with the wisdom that we have at that time.