don't believe him/her
Lo que sea que él diga, no le creas.
Whatever he says, don't believe him.
No le creas a la gente que dice que aprendió el francés en tres días.
Don't believe people who say they learned French in three days.
No le creas a la gente que dice saberlo todo.
Don't believe people who claim to know everything.
No importa lo que él diga, no le creas.
No matter what he says, don't believe him.