How to protect PPOR from renting defacto in case of break up ?

Hi everyone.

Apologies if this has been covered a million times before, but i couldn't find any helpful information using the search tool.

I'm posting on behalf of a mate of mine who has just bought his first house. He is living in it with his girlfriend of 6 months who lived together with another couple in their last rental. The house is solely in his name and the deposit is %100 his. She's paying $100 a week in rent and hes made clear that the payment is for rent just like any other friend and not a mortgage contribution ( which is how i personally believe it should be, having a girlfriend should not cost you money every week )

He wants to know if theres any way to protect himself from claiming any part of the house if they break up. Shes not the sort of person that would claim something that's not hers, but if she did her dads a lawyer so my friend is kind of screwed.

The other concern Is he even allowed to charge his girlfriend rent without risking losing his first home owners grant ?

I would much appreciate any advice.

Thanks