My name is XXXXX XXXXX X am a General Specialist. I would like to try and assist you. I have not sold shoes, but I love to buy shoes and I understand about the sizing dilemma, since I can wear a 3 in girls, a 4 in women's shoes or a 5 in women's shoes depending on the style and material of the shoe.
In a case like this with so many variables, I suggest taking a Picture of the pair of Shoes you want her to have and print it along with a Gift Certificate for the shoes and it in gift wrap in a box. This way she knows you have picked out the pair of shoes for her as a gift and she can use the gift certificate. She will then be able to get the correct size shoes that she will be comfortable with.
Shoes can vary is size because of the design and materials. I think this would be the best way for her to have the shoes as a gift without having to return because the size was wrong. Please let me know if you need further assistance.