Presumably you are asking whether the terms given on redundancy can vary between employees who are made redundant, other than on the 'accepted' grounds on length of service and age, if there would seem to be a sexual discrimination factor?
It's a very good question and one I've never come across before. Part of the answer on the acceptability of discrimination on 'age' grounds is given here - a publication originating from the TUC.
http://www.worksmart.org.uk/rights/doesnt_giving_people_different_redundancy_payments_based
But that doesn't cover the sexual discrimination question if, as Woofgang says, it is that.
Are you sure that this individual does not have a contract of employment with termination on a notice period of four weeks on either party? If so the employer has to give four weeks notice minimum, but this increases by one week for every year after 5 years continuous service (up to 12 maximum at 12 years) because the statutory obligation overrides the personal contract.