My sister is married to a guy and doesn’t want him anymore because he’s a liar and stuff like that. Now does he have to pay mahar or no even though he’s at fault and my sister is leaving him for a reason and his family and he himself know he’s wrong

She must consult with the mujtahid or his representative before taking any decisions.

If the mujtahid or representative rules that the man is at fault then he has to pay the Mahr if not then he is not liable.