I think your gf is being too ambitious. Even if you can just
afford it, you need to save up $$ for rainy days too. Try not to
stretch your loan to the max.
Sit her down, and help her to see the whole picture. Buying a
house, it's not just the cost of the deposit, you need to spend on
furniture, utilities (phone, electricity, gas, water, internet),
food, insurance etc.
Money problem is one of the major cause for relationship breakdown
including divorce in a marriage.
If you're not comfortable with the whole thing and know that you
can't afford it, DO NOT commit yourself to it.
Good luck! 
