> Special-K, I've been researching a little and what I find out it's not really nice

There is a lot of information on the internet... Some of it is helpful and some of it is judgemental.

Intent needs to be assessed and not assumed.

> Sometimes it's like she can refrain herself of the lie, it just comes to her mouth and she needs to say it, even when everybody around knows is a lie.

What sorts of lies does she tell?
Has anyone pulled her up on the lies in front of you?
How did she respond to that?

> It seems that the main reason for soembody to do this, is to gain power, it can be also for money or sex....

What sorts of lies does she tell for you to think she is lying to obtain those things?

It might be about... Her feeling insecure. Like she needs to lie to get her needs met. If that is the function of her lying (and intention needs to be carefully assessed and not assumed) then it sounds like she could benefit from learning alternative ways to get her needs met.

If you tell someone to 'stoppit' without providing alternative ways for them to get their needs met then it is likely to leave them feeling pretty bad about themselves and isn't likely to help them particularly...