The path back to work

Not sure if anyone has experience on the benefits system.

A friend currently works 10 hours a week and as well as other benefits gets her mortgage interest paid.

She has an offer to increase to 20 hours a week, but at £7 / hour this will not net her as much as increasing to only 15 hours a week as once past 16 hours she stops getting her mortgage interest paid for her.

She hopes this is a path to fulltime employment so is keen to take it but worried that she will be worse off in the meantime.

Any thoughts welcome