I don't charge for stats, just not done in my area. I don't charge for days when my services are not available.
If you contract says you charge, then you charge. I've had bad experiences with previous clients, that doesn't mean I charge my new clients more because of it. BUT, if you have told her she doesn't have to pay, then you should stand by your word. Have her sign a one year contract that is corrected to reflect the change. Next year have her renew her contract, but with stats paid. I don't like doing things differently for one family, if the other families found out they would be annoyed.