Is the the db scheme contributory?
In my company, in circumstances similar to this, deductions will continue to be made at based on the full salary, the employee gets full credit for the years spent part time working and his pension is based on a proportion of his final full salary.
If your friend made contributions based on the reduced salary, Im not sure how his service and pension would be calculated.
Your friend should get his hands on the rules of the pension shceme from his HR or Pensions department.
aj