Oops when your daughter receives the letter from the Benefits Agency, it should contain all the information as to how the overpayment occurred: what benefits are involved, time periods and the weekly/monthly amounts. She needs to sit down and carefully check every item - if the BA have made historical errors in her payments she needs to double check that theyve got it right this time. If the errors are the fault of the BA, she can appeal against their requests to have it repaid: if the fault/errors were theirs and not because she had provided false or erroneous information when she claimed, an appeal can be launched.
In any event, the BA are unable to demand that the repayment of any overpayments of benefits are set at a level that would put your daughter in penury: she couldn't, for example, be asked to pay £200 a week if that would leave her unable to pay her basic needs of housing, food, utilities and travel to and from work.
Do nothing now until that letter arrives and, under no circumstances be coerced into agreeing to a repayment plan without taking advice from StepChange or Citizens Advice. Feel free to pm me.