Advice needed - looking for input on what hourly rate my friend should negotiate for.

My friend works as the head chef for a venue that does wedding/event catering in central NYS. They have been with this location for 3-4 years now, and they are looking for advice before going into negotiations for a raise. They have a staff of only 1-2 soul chefs to help them, and the events range from 50-200 guests. It is high quality food all made from scratch, and they have never had a complaint relating to the food, in fact the guests consistently compliment it.

My questions are: What are some different standard pay packages people receive elsewhere for similar work? Or, does anyone have any good reference material to help give my friend a better idea of what is considered fair pay for this type of position?

Any advice or reference material is greatly appreciated!