I think that the FD derailleur cable originally was routed through the broken off guide, and then through the guide it currently goes through. So the first (now broken) guide gave you parallel cables on the down tube, and the additional FD guide allowed the cable to be nicely routed to the FD itself.
You could ride as is. Or grind off the stump of the old tube and braze a new one on. You could wait til you plan to repaint the whole thing, or you could just have the tube put on and paint the area damaged by brazing for now. Its on the bottom of the bottom bracket: you could just used primer if you wanted! Or grind all tubes off and figure out how to fix the plastic setup on, probably by fitting a piece of steel into that cutout