If the ex-boarder takes lessons, then I would want payment up front whenever the credit for lessons you allow runs out. I would worry about further payment, and I wonder if the boarder will only come to you for lessons as long as there is credit left, and then bail on you too.
However, I wouldn't do the lesson credit anyway. You had a boarding contract, and it calls for 30 days notice. I think you are being generous about not making the boarder pay more than one month.