She's a senior and she's done for the year. But let's say she does live up to her commitment and goes to sectionals and the team makes it to state. What if the girl was enlisted to go into the service right after graduation? Or begin summer college courses? Or a job? Should she also put those off so she can finish her commitment with the team?
If the OP is going to be mad at anyone, be mad at the coach who allowed a senior to participate in a spring sport, knowing full well the sport could run after graduation.
Most employers wouldn't give an employee who they know is leaving the company soon a big project. If anything they tend to reduce responsibilities the closer the exit date becomes for the very reason that right or wrong, when people know they're on their way out, they tend to mentally (or physically) check out.
For all anyone knows the JV girl might surprise everyone and do better than the girl leaving the team. And I'm sure she'd be happy to know what little faith her future team mates (and their parents) have in her.