This is the 13412th most frequent Russian word.
наивно
Naively.
In this sentence, 'наивно' describes the manner in which he thought, indicating an unrealistic simplicity of his assumption.
Он наивно думал, что всё будет просто.
He naively thought that everything would be simple.
Here, 'наивно' serves to characterize the belief as unrealistic and simplistic.
Считать, что все люди честны, наивно.
To believe that all people are honest is naive.
In this context, 'наивно' reflects the innocence and lack of understanding in her response.