Contact with pitching plate

[mg43]

I just started coaching in an area where fast-pitch softball is relatively new. My 12 year old pitcher uses a slingshot style delivery whereby she drives off of the pitching plate with her pivot foot to gain most of her power. Her momentum, of course, moves her toward the catcher and she drags the pivot foot forward 6 to 12" each time. The pivot foot never leaves the ground. My opposing coach argued that it is an illegal pitch because her pivot foot does not remain in contact with the pitching plate through the time at which the ball is released from her hand. I'm a newbie but I have watched high school and college windmill pitchers leave the rubber before the ball is released. Tell me if I am missing something here. I had a difficult time defending my pitcher even though I'm fairly certain that I am right. Hopefully I'll find the rule in the ASA book.