Is your girlfriend working full time? Under the current rules for Mortgage Interest Supplement, you won't qualify for MIS if she works 30 hrs per week or more, regardless of income.
Are you the only name on the mortgage, or is your friend still a co-mortgagee? There are examples on AAM where couples have separated and the person remaining in the family home can't get MIS until the house ownership has been resolved, so you might have similar difficulties.
Your best bet is to speak to a Community Welfare Officer - they are based at your local health centre.