What help can sons girlfriend get to get through collage?

my sons girlfriend moved back with her mother last year after living with her grandparents most of her life. Her mother lives a 2 hour bus ride from her school so I agreed she could stay with us during the week to enable her to finish school (she was 15 then). During this time her mother has not supported her financially at all. We have been left to pay for everything food, clothes, transport even her prom dress etc.Her mother is on benefits and claims everything going but when she asks for money for the simplest thing her mother refuses. She has now turned 16 and wants to go to collage but her mother refuses to support her financially in this and we can no longer afford to do this. I am worried that all the hard work she has put into her schooling will be wasted. I could report her mum and see about claiming the money myself for her but dont want to cause problems for her with her mum.It is really annoying that her mum is sitting back raking in money and not spending benefits on what they are intended. It really needs sorting soon but i am at a total loss at what to do...any advice appreciated