He already did in a way. Franky punched Luffy, to send him away from Saturn's attack. Saturn can make people's heads explode and prevent them from moving just by looking at them. Perhaps he uses an invisible spider's net, or some sort of telekinesis. Doffy made Riku attack his own citizens with strings, Saturn could perhaps do this too with spider strings, or through mind control. Luffy fought a Strawhat in Whisky Peak (Zoro) and again in Water 7 (Usopp). Now he is so weak that Franky attacking him would be very dangerous. Sanji was susceptible to Saturn's powers, so they should work on Franky aswell.
