My A/C unit wouldn't blow cold air about a month ago, a friend came out and said we needed more free-on(?) and so he added more, now my unit outside makes a loud squealing sound and my furnace inside has been leaking water for at least a week. What is the problem?

Could be to much freon/refrigerant was added. I would have a REAL professional go over your system before more damage is done.

How did your friend know you needed more refrigerant? There is a procedure for adding refrigerant and if that was not followed damage could be done to the compressor.

Was any attempt made to find and fix the leak? I fear you will have to start all over again once the added refrigerant leaks out. When that happens insist the leak is found and fixed.