A week 1 basis means that you receive 1 weeks worth of tax credits and standard rate in a week. So if she received 16 weeks income in one payment she will have only one weeks credits and amount at the standard rate.
She should be able to get Revenue to issue a cretics of Tax credit on a cumulative basis. Thats why revenue have been writing, to prompt her to submit whatever they need to do that. basically state that she does not have other income absorbing the tax credits.
If she doesn't do this it will be next year before it is sorted.