Choosing a suitable birthday gift for your girlfriend, Shaina, who is on a diet involves considering her health-conscious choices while still making her feel special. Opt for gifts that align with her diet, such as a basket of fresh fruits, a selection of gourmet herbal teas, or a cooking class focused on healthy recipes. You could also plan a day of active and outdoor activities like hiking or a yoga retreat to encourage her commitment to a healthy lifestyle. Additionally, a heartfelt message expressing your admiration for her dedication can complement the gift and show your support.