That が is a subject marker, and the corresponding predicate is omitted.
A: だって8割はホントだもん。
B: へぇー 8割本音を言わない男が（8割もホントのことを言うの／そんなに本音で喋ったの）?
The person who doesn't say his true feelings 80% of the time (says that much truth this time)?
But it would be too wordy if you tried to translate this literally, so your translation attempt seems fine to me.