So it's a bit much that he expects her to pay 50% of his mortgage, plus 50% of bills, food etc, and yet expect her to sign a document (probably with no legal substance) that says she can never have any "interest" in the value of the property. Any such document would carry no legal weight anyway, it's like a landlord demanding access to a rented property with 24 hrs notice, completely unenforceable.
Of course a legal document renouncing a non-entitled spouse or partner's rights would be of legal effect. Its a standard document that mortgage providers usually require to be signed.
And if you are in Scotland, you are mistaking the law relating to economic interest in shared property when a relationship breaks down. That is based on the concept of economic disadvantage. In this scenario, she would not be able to prove economic disadvantage, because if they split up years down the line, she would not have been economically disadvantaged unless she could prove she had outlays relating to property in the relationship which significantly outweighed those she would have had to pay to live otherwise. The only couple of cases which succeeded under that law involved partners who had sold their own properties to invest in those of the other partner to the relationship.
In other words, she would still have to pay rent and bills or a share of wherever she lived. Its not considered an economic disadvantage to do so. The law doesn't presume a partner will provide for another financially, particularly not after a 6 month relationship.
Some mumsnetters also seem to think that anyone who pays rent is paying someone else's mortgage. They forget those inconvenient things such as saving up for a deposit, meeting the financial checks for mortgages, conveyancing fees, stamp duty, furnishing, etc.. Plus a mortgage might cost £450 a month (mine does on a similar sized property) but rent for £900 plus a month. But the buying process and deposit might have meant an outlay of £40,000 to get that mortgage payment. And if the roof or central heating needs fixed, guess who pays?
I do see women who do this - they tag onto some man with a good job, a bit older, and move into their property. Then if they are lucky the relationship works, if they are unlucky it breaks down in time and they have to rent a room in a flat somewhere - that's their choice. Other women, like me, choose to buy our own properties, even if we met a man with his own house and moved in. Its not exploitation by the man - its more like wishful thinking by the woman that some fairytale will mean they don't ever have to face up to the realities of life and sort out their own property to live in. That someone they barely know will just give them a half share in a property without any financial contribution beyond paying their own way in life. Its ridiculous.
I use the words "men and women" but I know men who do this too. I'd hate to be in a relationship so unbalanced, it would feel all wrong.
In this example, theres nothing to stop this relatively young woman from continuing to work, save up her own deposit and buying her own property in time.