Hey efast let’s discuss the front knee.
Should her knee be doing something different than what's shown in the video?
Ok, what I see is her front hip moves away from the pitcher as her front knee straightens. Just about perfect. The fact her knee rebends after contact may result in some deduction in style points but it matters little because the ball was crushed. Oh wait, there is no style points in softball. She almost has a little scissor action going on there. She is not one legged and has a noticeable slide/weight shift. I have no idea who this is but this is a nice swing.
Looks like the back leg is in control. The stride is soft, a little reach, stride to toe touch keeping the foot from planting. Good launch position. Weight is always back at launch.