NUT Strike action 30 June

Great news that the NUT are taking industrial action on the 30 of June but have been approached by several Unison member I know who work in schools and asked whether they will be right in refusing to cross an NUT picket line at their school? I think the answer is that they are within their rights not to do so but can someone from the NEC clarify this? I have send this message to Jon and John who are now both on the NEC and will see what their answer is if any.