Mandon moore says your opponents characters can't be saved. Is the red viper immune or does my opponent not have the option to even choose the viper to save him.

Mandon Moore is a character ability that works on characters, making them unsavable. The Viper therefore ignores the effects of the character's ability. Any particular reason or argument someone was giving for why The Viper would NOT be immune to this character ability?

Of course, The Viper's immunity eliminates almost all saves other than dupes and Iron Mines, so it's not much of a strategic loss for Moore's controller, I would think.