Hi --- When a deposit is paid towards a cash purchase on a car, it is not an automatic right to demand your money back unless you could prove that there is a fault with the car, which could be costly.
Why does your friend not compromise and tell the dealer she will go ahead with the purchase, only if the dealer provides her with a full written guarantee free of charge against any defects or problems arising with the car for at least Six months, this will at least show a degree of goodwill..... that the dealer stands by his word that the car has not got a blown gasket or is not developing a head gasket problem to his knowledge.
I can't really see him trying to sell it in that condition as it would all come back to haunt him anyway, if he sold a car in that condition.
PS -- general cylinder head gasket problems are usually more obvious by a sluggish lumpy response, lack of power and White smoke coming from the exhaust.