it''s still an outgoing for her, regardless of how little it may/may not be. I would therefore think it appropriate it be listed as one of her liabilities. It doesn''t necessarily mean she''s looking for it to be paid back as part of the divorce process or that a judge would take it into account.
It is true that repayment levels are low but they increase with salary earned. If she is therefore on a ''good'' salary, it is unlikely she is paying at the lowest rate. In any case, it should show up on her bank statements somewhere which you should have copies of if you have exchanged your form E.
Student loan repayment rate is 9% of salary over £15000 (for loans between 1998 and 2012). You can therefore calculate her salary to be approximately £17500 pa. (£18 x 12 months / 9 x 100 + £15000).
It is almost invariably deducted at source (through PAYE) so probably wouldn''t show on bank statements but should show on payslips. The only exceptions I know of to this are where the person doesn''t pay tax through PAYE; i.e. the person is self employed or where they live abroad etc.
As others have said, she may be listing it simply to show an outgoing rather than a liability for the debt itself.