The reason that the frame is sweating is because the window is only a single-pane one and the metal does not have a thermal break between the inside and the outside. The metal brings in the cold temperatures and moist indoor air condenses on the metal surface. The only solution to this problem is adding a second window. If you want to do a temporary job, you can simply cover the window with some plastic sheeting, but the plastic should be at least 12 mm (1/2 in.) in front of the metal, or it too will get cold. Ideally you should add a second window, or change the whole thing for a modern energy-efficient window.