Good starting point would be to speak with the company who provide you with your respiratory equipment as well as MSDS. Depending on the duration, ventilation etc respiratory protection may vary. If the fumes can be extracted from the source stopping the person breathing in the fumes you may not need to use face mask, which will be decided by your risk assessment.

My first question is 'what is the process being used to cut the steel?' Only when we know that can we assess whether there is going to be zinc fume, or not.