Thalia,

Good for you for working on boundaries.  Insofar as dealing with your co-dependency, I recommend checking one or more of the vast library of books and websites around that.  There are also support groups for that as well, CoDA.org.

How could I work on this without dissolving the friendship any?  Put value on your healing first.  You assume that this will end the friendship.  Even if you part ways, nothing is permanent.  You may be able to reconnect as you get more healthy.  I am no expert but I suspect that even your question is a co-dependent concern.

Mark