I'm not saying anything about the foot being on top of the rubber. I'm saying prior to pushing off her heel will come up and she'll lose contact with the pitching plate. She'll be illegal unless she's flat footed and maintains contact while pushing off and nobody I've ever seen pitches like that.
Once again, this has been covered several times in this thread. Please read posts 7, 9, 14 & 15. If the pitcher is in legal position for the start of the pitch, she is considered to be pushing off from the pitching plate.
Here is a cut and past of MTR's post with the rule.
6A.1.E.2. (Female) Both feet must remain in contact with the pitcher's plate at all times prior to the forward step. If the pitcher's pivot foot is in contact with the PP at the beginning of the step, she is in compliance with the rule in pushing away from the pitcher's plate. 6A.3.J&K