I'm with you Ant, although I also agree with Alps... after taking info from here go and see a solicitor.
BUT
Beware - if the solicitor smells the money, you will spend some.
To be honest your mate has played it all wrong.
Until you have kids with them, whats hers is hers and yours is yours until then keep it all seperate and if she lives with you no bills or loans or owt like that in her name, just cash each month for rent/bills/food.