Arthurg .. the correct term is giro postal .. I guess since these are most often purchased at the post office ... A bank transfer done from the US would not be difficult. We do it often from here. I beleive it costs around 35 dollars at your local bank. All you would need from her are her "20 digitos".
We Americans would need to be tortured before we would give up our bank account numbers but it is often a common thing in Spain.. I was just asked for it when I signed up for a new cell phone at Vodafone ...
I would think however that she might just want cash .. Why dont you ask her what she would prefer?