Ingenious means "inventive," "resourceful," "talented," "imaginative." Ingenuous means "naïve," "frank," "unsophisticated," "artless." "Alex's suggested solution is ingenious." "She is an ingenuous little child." Naïve (correctly spelled with a dieresis over the i but also considered correct without this mark or spelled naif) has much the meaning of ingenuous. It is applied to persons who reveal natural simplicity, artlessness, and innocence. Naïve usually suggests traits that are amusing to the user of the word but not to the person so described. Probably no one would like to be Diane in a statement such as "Diane is so naïve she believes everything her boyfriend tells her." See also unsophisticated.
Dictionary of problem words and expressions. Harry Shaw. 1975.