The English expression for this will be "Take what she says with a grain of salt", that is, "She does not exactly/entirely tell the truth (,so take it as such)".
It is possible to say "I will take what you say with a grain of salt" but it would be rare or rather challenging, basically saying in one's face "I do not entirely trust you".
The structure of the sentence is:
- [What X says] = [X の言うこと] (in this sentence, X is 'she')
- is not entirely from
- [true feeling/thought of X] = [本心]
[What X says] ≠ [X's true idea/feeling/thought].
It is possible to replace X with 'You', but again to say so in the face of someone would be too direct. It is possible to replace with 'I' but basically saying "I do not entirely tell the truth".