Right action when being lied to

That is too vague. It depends on probable or possible consequences of the lie. Whether you will have to rely on or cooperate with them in future. And what harms and suffering may arise from their behaviour repeating or remaining unchecked.

The best thing is to be really present with them when discussing challenges like this, and try to open yourself to seeing how their mind works. Potentially then you can find a direct way to communicate the issues to them, as though you were them and they were you.

In Buddhism, the best action is always the same: be compassionate. If someone close lies to you, they are afraid of something or wanting to preserve some element of their ego-identity. You don't have to know why they are doing it or what the truth is; you just need to accept things as they are. Keep your eyes open in case the lie they are telling causes them troubles so that you can help them out of it, but otherwise allow the situation to play out as it will.

If you allow your ego to get involved — becoming angry because you aren't told the truth, becoming insecure because you feel like you're on the outside, becoming prideful because you think you see through the lie being told — you'll just aggravate the situation. Nothing needs to be done until something needs to be done.

Everything that happens to us is caused by an action we have committed in the sansara. This was common to Buddha as well (Devdatta's actions etc.).

Therefore the answer to the question What is the best action? is, Understand that this is a result of an action you have committed in the past (maybe in previous births). Further, try to spread metta (kindness, love) towards him though he is lying to you. In future, he will admit what he did today. Until that there's no much meaningful thing to do.

Welcome, Lineke! I don't think that there is a straightforward answer, as I think the situation deserves some analysis. I do not claim that this is the "best" action; I am only using what I've learned from a few Suttas as a guideline, so I would encourage you to read more on the matter.

First, I will assume that you know for sure that you are being lied to. Second, I am assuming that, when this other person lies to you, you feel hurt, or angry, or betrayed, or it causes you to suffer in some other way (I am assuming this, because falling into this kind of distress is only human, as we all know from first-hand experience). Now, with those two assumptions in mind, I put forward that the problem has two elements: the harm the person is causing themselves by lying, and the harm that they are causing you with their actions. Please bear with me, as I think that addressing the first is important to be able to address the second part.

Regarding the first part, the Buddha taught that we are all heirs to our actions, and have to live with the results, good or bath, whether in this life or the next. This means that the person lying is generating bad results for themselves. At the very least, they'll have to live with the lie weighing on their conscience and, depending on whether one adopts the view of rebirth, they aren't helping themselves to secure a good future life.

Now, addressing the second part, a relevant Sutta that touches on how to react when one is being harmed is MN 21. In it, the Buddha explains that if a group of bandits should capture someone, and savagely cut them up, limb by limb, even then that person should cultivate feelings of goodwill towards the bandits. Extreme though that simile may be, it illustrates an important point about what the Buddha thought a good strategy for dealing with harm is—namely, to cultivate goodwill towards everybody, regardless of whether they are being harmful to you or not. Now, as a lay practitioner, who is as human as anyone else who is not an Arahant, I know this is incredibly difficult, and perhaps you'll realize that in order to cultivate goodwill, you will have to let go of a few things.

If you find that cultivating goodwill for that person is too hard, then perhaps you can weather the issue with compassion, or equanimity. When one considers that the person lying is causing harm to themselves, one can maybe begin to feel a little compassion for them. If one understands that the harm they are causing themselves will only make their life worse, one can begin to feel genuine concern for their wellbeing, and not in a condescending kind of way. This is, in fact, one of the strategies the Buddha taught for dealing with feelings of animosity toward another. If, when you feel anger, or hurt, or betrayal when you're lied to, perhaps remembering that the person doing the lying is harming themselves will help you let go of that suffering and develop more wholesome feelings instead.

I hope this is helpful. With Metta.