The fact that they haven't work one of their scheduled M-friend shifts doesn't give them the right to work Saturday. By the employee choosing to not come to work or being sent home as a no pay doesn't guarantee them 40/5 days by working Saturday. I can assure you any FT with less senority than me (whether they worked 5 or 1 days this week) would not be working, if they did it's easy money for me as management would be paying an easy grievance for all hours worked by said employee.
You have to remember that Saturday is a air day, as such PT employees have precedence over FT employees. If a PT cover driver runs ground M-friend they are at the bottom each day but on Saturday they have senority over any FT and have first dibs to work. They will receive their air rate on Saturday for all hours worked. When the company runs out of PT employees, then the work is offered outside of that classification to FT, in senority order.