The buck actually stops with the captain : if one chooses to ignore advise like this and something goes wrong no amount of delegation will change the fact.
This looks like either someone attempting to save their credibility through the courts, or there is a subplot we are not aware of here.
Either way, once the warning is received it becomes a no brainer firstly in terms of safety being paramount, and secondly should there be an issue regarding SOP it is for the company to sort out not the flight deck crew... there are tea and bikkies for cabin crew too.
I'm surprised this has got so obviously out of hand as it apparently has.
Certainly, many aircraft types have an allowable frost rider to the clean wing policy, and those caveats are usually very specific in terms of location and depth of deposit.
CRM applied, de-icing completed, no accident occurred.
There was only one decision to be made, and made it was ... and somehow, ludicrously, this ends up in court.