Frelinghuysen is also just a person, he wrote a personal note at the bottom of a campaign flier to a registered supporter. There was nothing illegal about that.
So many people are fast to support the first amendment when it comes to Liberals attacking Conservatives but when those Conservatives fight back and exercise their first amendment rights to voice their opposition to unfair attacks suddenly nobody wants to admit it is the same both ways.
She had her right to attack him, he had his right to expose her activities to her employer. If the employer now suddenly informed about her political activities felt her behavior was not in the best interests of their company then that is between her and them to work out.
It seems she was hiding a lot of her activities from her employer specifically because she knew they would not approve so she was digging her own grave intentionally. All Frelinghuysen did was share a fact with her employer that she was a major player in a political movement.
If you looked into her Super Pac funding how many of her donors were clients of her Bank? I bet that is the real reason she quit, ethics questions were going to blow up in her face.