If someone is intentionally deceiving someone else and if I am aware of it, if I inform the innocent person in this matter, would that be against five precepts?
It would not be against the 5 presepts in the Theravada tradition.
There could be a better skillful way to approach the situation. For instance:
If someone was trying to sell your friend a car and he tells your friend that it is in pristine condition but you happen to know that it is really held together with bubblegum, then you could remind your friend to test drive the car.
However, while your friend is distracted, the salesman has his security guards escort you off the property and guard you from coming back and alerting your friend.
Anger arises in you but after you practice Metta for a bit the lay buddhist guards notice the difference in you. While your manner is drenched in metta, You simply state what the hell realms are like and you simply state what unwholesome transgressions one can do if they want to go there.
Hearing there own unwholesome actions in the list, they let you go and they make sure their boss doesn't go to hell by simply stating that if he lies all the time he will most likely go to hell all the time.
So now your friend is free to buy a real car.