Save your indesign cs6 file as IDML, she can open it on CS5.5.
Thank you for your answer, but i have heard that it doesn't save all the file, that it may some things missing, is it true?
Just try, and compare between 2 files.
Is the OS system as the same, is fhe fonts as same?
IDML is the standard way to save a version openable in versions as far back as CS4.
However, any new features in Cs6 won't translate backwards - so it's a good idea to give a PDF with the file to demonstrate the layout.
An example of something that wouldn't translate backwards say from CS6 to CS4 would be the Span/Split columns, as that feature did not exist in CS4.
The other thing that gets affected is the "Text Engine", you may have different line endings for your text, this may or may not result in text overflowing.
This is because the text engine is updated each version which can cause text to shift slightly within a line or paragraph.
Your friend is not legally allowed to "give" you their software.
Have her upgrade to CS6 if you really want this to work. And if it's that
important to you, then pay for it as a Creative Cloud month to month until
you finish the project or buy the upgrade.