There are two types of Pension Credit.
Guarantee Credit is there to to ensure that the income of older citizens can't fall below a certain level. (i.e. it provides a 'top up' to any other income that the person might have). So if the person's income rises, the Guarantee Credit will fall by the same amount, since the minimum income level will still have been achieved.
Savings Credit works slightly differently but the best way for your friend to assess what she's entitled to is to use the calculator which is linked to from here:
http://www.direct.gov...ow_income/dg_10018692
Chris