How would you repair this copper drain pipe, and how much pipe would you replace?
That's a bad spot to have a leak. That could have been caused by too much heat from a torch or excess acid flux. You might be able to cut the 1 1/4" pipe on the other side of the support and heat the joint going into the "y" and remove the piece of pipe. Then clean out all the old solder by heating up the pipe and wiping out solder. Use emery paper to clean the surface down to the copper. Get a new piece of pipe and a repair coupling. Slide the repair coupling on the old pipe after fluxing and then slide the pipe into the "Y" after applying the flux. Flux the new pipe and slide the repair coupling into place. Now sweat the "Y" and the repair coupling. You should be good to go.