There are a lot of threads on this site and others about the merits, or otherwise, of parents using technology to oversee teenagers' behaviour.
They usually deal with such issues as:
What will you do if you find something you don't like?
How will she react when she finds out you're looking in?
If she knows you're looking in, might she do her "secret" communicating via other means?
It's possible that the best solution to the problem may not be technological.
Best wishes and good luck.