Are you talking about making a friction weld on a cnc lathe, or cutting off material from a part that is already friction welded?
If you are talking about using the lathe to friction weld a part, I have done it on a manual lathe, but I'm not sure that I would try it on a cnc lathe. You have to stop the spindle almost immediately after the material reaches the melting point. I'm not sure that the decel rate on the cnc lathe spindle will allow it to come to a stop that quickly without damaging something.
It does work on a manual lathe with a footbrake for the spindle. I've only used it to weld together two pieces of bolts to make a long bolt.