How would one ‘unhack’ or ‘untrack’ a phone, anyway?

I did not honestly think that my first post in years (or has it been one year- I can’t remember) would be a question, but hey, I’m not really a phone person.

A friend of mine has told informed me that her phone (seen here: http://g2.t-mobile.com/g2x) was either hacked or tracked (likely both, considering that the individual responsible has been providing her with real time information on various activities). Now, while I have some technical ability, none of said abilities involve mobile devices- being completely uninterested in smartphones. This, unfortunately, means I’m completely out of my element in dealing with this situation.

And so the question:

What would be the best method of resolving this situation?