Limit Multi-select picks to one in linked table

There is no native capability to conditionally filter a multi-select type field. You could do this if the functions were child records.

The only other option would be to add the same multi-select field to the roster allowing you to specify which functions are needed for that shift. Then set up a way to 'check' the functions on the roster against the employee's functions and prevent the record from being saved if there is a mismatch.