Was your father the/an executor of his friends estate and he left 6 years elapse before applying for probate?
The valuation date is normally the date of grant of probate.
However the date of benefit is the date used for calculating the tax due: usually date of death is date of benefit.
"The legislation provides that the relevant group thresholds, rates of tax, reliefs, etc. used to determine a beneficiary’s tax liability are those pertaining at the date of benefit and not the valuation date."
and
"the valuation date of an inheritance is normally the date the administration of the estate has been completed."
from
a google for "date of benefit" site:revenue.ie
and go to the CAT manual.
As the thresholds have gone down since 2005 this has worked in his favour as valuations have tanked