TCL:
The 'pro rata' bit means that, if either party had taken up the option to cancel the contract after the first two sessions, DonaldDuck92 would have been entitled to a refund equal to what she'd paid up to that point minus two elevenths of the total agreed for the full contract (assuming that she'd already paid for more than two sessions). If, at that point though, she'd paid for less than two elevenths of the full amount, she'd have had to pay the excess required in order to bring her payment up to that level.
It's entirely irrelevant now anyway, as the 'two elevenths' point has long since passed.