I understand the method is not the end all... but I have to have some type of reference point. Because of her weight/size and skill level I think a 138 would be the right size for her, but thats my opinion
A 138 is too small. Many years ago, when I was 120lb's, I was riding a 146 board for park. As Chomps mentioned, the height thing is the absolute most terrible way to measure how big a board should be on someone. Also, to my knowledge, almost all self respecting manufacturers will use weight as the guide, not height.