No if he/she is not being hired to do the job. As long as the photographer is working and is receiving compensation for the job then a permit is needed.
What tons of people try to do is "smuggle" the photographer into their weddings saying that they are family members. But the immigration people is like the cops, they can "smell" if there is trouble going on.
Everybody is different, so everybody can take different levels of risk. Also, every location has different immigration people working on the area. What I would do is talk to my venue or wedding coordinator and see what their recommendation is. They would know if they get supervised or not and the level of supervision they get.
In case they recommended getting the Work Permit just call or go to the closes Mexican Consulate in your area and ask about it. Maybe the photographer could even apply and get the permit upon arrival into Mexico.