Our company's deicing manual states that with a SMALL AMOUNT of frost/hoarefrost is acceptable as long as you are able to see the color of the paint through the frost itself.
Usually on the underbelly of the wing by the fuel tank, it's fairly common and acceptable to depart in this condition.

Again I said a SMALL AMOUNT of TRANPARENT Frost in which the color and body of the aircraft's skin can clearly be discerned. If the frost's thickness exceeds the point of transparency and becomes a translucent opaque then it the aircraft should be deiced.

Additionally, large amounts of frost or any other type of frozen/snowy accumulations on the aircraft also merit a visit to the deice pad.

If any doubt exists or the wing is questionable at best then by all means deicing the aircraft is the appropriate procedure.

That being said I don't agree with the flight attendants reporting the incident to the FAA, why......Because the Captain and the First Officer did not do anything wrong. The Flight Attendants were concerned about the frost so the flight crew went to the de-ice pad. You can't violate the flight crew for something the MIGHT have done.
Had they refused to deice to the point were the flight attendants refused to fly then I can see a major issue unraveling, but that didn't happen.
Flight attendants raised their concerns and the pilots remedied the situation.

Granted the FO was careless in his walkaround, but again that is an Inter-Flight Crew issue which should have been mediated and remedied by the Captain.

If there is a personal aspect to this entire lawsuit, then it wouldn't behoove anyone not to defend or start a grassroots campaign to help either side.