F/O definitely inappropriate remark.
However.
Perhaps the captain should have brought up the situation of storms along planned route with the FO as maybe the FO hadn't noticed it then seen what the FO was thinking if it was nothing unsafe and was within SOP's why not let the FO divert when he likes, or perhaps the captain could offer up an alternative and say something like "did you notice such and such aswell, the winds up here are from x direction and if we divert now and to the left it will be a smaller heading change and we are also diverting upwind of the weather and therefore we'll have a smoother ride etc"
From our CRM where I'm currently employed basically says if the FO is PF then they are responsible for the flight path management etc, and if they want to do xyz and it is within SOP's legal and safe then why not. Also states that the captain always has the FINAL say regarding the decision, and that the captain if he/she disagrees with the FO course of action should offer up alternatives first before pulling captains rank.