There are a few ways of approaching this problem. You cam try to cut every few stitches from the back using your seam ripper and then pull up the other thread. Or you could try to 'skin' the quilt. You lift the backing and carefully cut the stitches using a seam ripper (I have never tried it with anything else but I suppose a very sharp small scissors might work). It is fixable. Just take a few deep breaths, forget who this quilt is for(because emotion can derail all your efforts) and fix your oopsie.