Just go to the requirement of the position: As per example ....
Main duties
Restaurant and food service managers perform some or all of the following duties:
* Plan, organize, direct, control and evaluate the operations of a restaurant, bar, cafeteria or other food or beverage service
* Determine type of services to be offered and implement operational procedures
* Recruit staff and oversee staff training
* Set staff work schedules and monitor staff performance
* Control inventory, monitor revenues and modify procedures and prices
* Resolve customer complaints and ensure health and safety regulations are followed
* Negotiate arrangements with suppliers for food and other supplies
* Negotiate arrangements with clients for catering or use of facilities for banquets or receptions.
Employment requirements
* Completion of a college or other program related to hospitality or food and beverage service management is usually required.
* Several years of experience in the food service sector, including supervisory
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He has to place the Reference Letter covering above duties during his experience in his employment ...
Just get the letter on the Letter Head mentioning his duties matching at least 70% of above requirement. You have to mention those in bullet pointed ..., you will get all the information in the side what a Reference letter should carry ..
All the best