Person A: だって８割はホントだもん。

 

Person B: へぇー　８割本音を言わない男が？

So little context to be sure, but my best guess is Person A is arguing what the 男 have said or done is 80% based on reality and Person B jokes by saying the man usually says only 20% of his real thought.

As for the が, what is supposed to follow it is "８割はホント(の事言ったの？)" or something like that.

Person A: だって８割はホントだもん。

Person B: へぇー　８割本音を言わない男が？

So little context to be sure, but my best guess is Person A is arguing what the 男 have said or done is 80% based on reality and Person B jokes by saying the man usually says only 20% of his real thought.

As for the が, what is supposed to follow it is "８割はホント(の事言ったの？)" or something like that.