I do stained glass and unfortunately, the only way to repair it is to remove the broken part and replace. There is a way of using copper foil on the top and bottom of the break and creating a faux lead line but I'm thinking that considering the piece, it would also be too pricey a fix. You could ask your local hardware store or go online for glass glue. It is made specifically for cracks in glass, though usually somewhat less extensive than this and hardens in the sun (ultraviolet). Personally I follow the advice of an earlier poster and turn that side toward the wall.