As has been pointed out, 27" wheels and tires are still readily available. But if your girlfriend will be using the bike for a long time still then it might be better to see if the brakes can be adjusted down a little to take 700c wheels instead. That way she'd have a wider range of tires available. You only have to be able to move the brake pads down by 4mm (about 1/6 of an inch) to fit the 700c size instead of 27".
lets be realistic, its hardly ever that easy.