Hi Sassy Gal :)
There are three open way. First just forget about rent and continue with your friendship. Second never forget this deed from him and cut all the relations with him/her. Third just gather all the proofs and document and sue him/her and fight till you win.
All these options depands on which sign you have.
Most probably if you are Aris, Cancer, Libra or Capricorn you will take the third the hars action and you will win, it is notmally seen in Char or movable sign tendancy.. If you are Taurus, Leo, Scorpio or Aquarian which are sthir or still or unmoving sign,you will select the first option to forget and continue with him/her. But if you are Gemini, Virgo, or Saggitarious which are multi tendancy sigh of moving ( Char) and still (Sthir) signs you will always be in this kind of dilemas for "What to do" or just " Do or not to Do" like Othelo of Sexpear :(
So just be still and be practical and just warn him and get payment which ever is recovarable and continue with him/her as friend but with warning in mind about this instant.
Rest, Do it your self quckly :)