I'm sorry to hear of this with Duke. Intestinal lymphangectasia is usually synonymous with PLE. This is what is recommended concerning diet in dogs suffering from lymphangectasia:
Ultra-low fat, easily digestible diets should be fed to minimize fat malabsorption and lacteal flow.
Cottage cheese (1% fat), no-fat turkey breast, and cooked white rice are excellent temporary options or may be added to elemental diets to improve palatability.
Prescription highly digestible diets containing less than 3 grams/100 kilocalories metabolizable energy from fat may also be acceptable.
Hydrolyzed protein or novel antigen diets should be considered in patients with concurrent inflammatory bowel disease.
Most prescription diets in this category are not adequately fat-restricted for patients with lymphangectasia; consider a home-cooked diet or one of the hydrolyzed protein diets that are fat restricted.
Gluten-free diets should be provided if gluten-sensitive enteropathy is suspected.
Elemental enteral feeding products may be helpful in very compromised patients. Such diets contain free amino acids to facilitate nutrient uptake by a compromised GI tract. Low-fat formulations such as Vivonex T.E.N. are appropriate for dogs with lymphangectasia.
Cobalamin (vitamin B12) should be provided if serum concentrations are subnormal. Folate supplementation if serum concentrations are subnormal. Vitamin and mineral supplementation may be necessary in dogs with lymphangectasia as uptake of fat-soluble vitamins is substantially compromised. Vitamins D, E, and K may also be depleted; supplementation of vitamin D must always be preceded by measurement (confirmation) and monitoring. Calcium and magnesium supplementation may be necessary.
Duke's vet can help you find appropriate diets as mentioned above. You also have the option of having a nutritional service help you such as here: https://vet.osu.edu/vmc/companion/our-services/nutrition-support-service when considering home-cooked recipes.
Please respond with further questions or concerns if you wish.