Quasi-legal here in the US.
§ 121.629
"Except as provided in paragraph (d) of this section, no person may dispatch, release, or take off an aircraft any time conditions are such that frost, ice, or snow may reasonably be expected to adhere to the aircraft..."
Who is to say the FO didn't walk on the wing, kick the snow around noting to the captain that it's just a light dusting and will blow off comrade.
Now careless or reckless operation...that's up to the locally paid off FSDO to decide.