Pension Guarantee Credit is paid to eligible couples to ensure that their joint income can't fall below £248.80. (There might also be an additional element of Pension Savings Credit but we can ignore that for now). Pension Guarantee Credit is paid to eligible single people to bring their income up to £163 per week. (Again there could also be a fairly small amount of Pension Savings Credit to add onto that).
If your mother is now single (i.e. separated) then the system should be looking at her income (if any), such as that from a private pension or from investments, and topping that up to £163 per week, rather than continuing to examine the combined income of your parents and topping that up to £248.80 per week. So, unless such matters have already been resolved anyway, that would appear to be what the letter is about.