I found published results of my mentor to be wrong. How to tell her?
As a matter of fact, I believe there is no such thing as guilt or ruin
for other people's work when it comes to the academic aspect of the knowledge.
Actually, if you do not share this with your mentor in order to be fixed, you 
may ruin many other further research...

I suggest you do this respectfully and carefully. You can go to your mentor and talk about this problem. You can show her your code and your understanding from the paper, then confirm whether or not you are right. Let her decide what she can do about this. There is also a slight chance you have made a technical mistake while investigating their work, so keep this in mind when you discuss the issue.