So broadly speaking: If you had a very good friend who was cheating on their partner would you be willing to back up your friend on their lies?

The two scenarios I'm specifically curious about:

You've had a friend for years, they have been in a relationship for 6 months. You know they are together, you know your friend is sleeping around behind their partner's back, and you know the partner thinks they are monogamous. One day while all three of you were together your friend lies and says they were with you the previous weekend, with out giving you any prior notice or warning that they've already told their partner this lie. Do you go along with your friend and lie about them being with you or is it more important to be honest and tell their partner the truth?

And same couple, what would you do if the partner got you alone and directly asked you if their partner was cheating on them?