Gosh, I've had quite a lot of dogs, but I've never heard of one who ate cucumber! I'll have to try it!
Our vet recommended that we gave our dogs 'homemade' meals of meat (chicken or pieces of casserole beef), rice or noodles, and green beans or courgettes. All cooked in a massive pan! He said the vegetables were good for their digestive system. I would sometimes add an egg (just break it over the other ingredients, towards the end of the cooking time, and stir), or grated cheese, and of course a spoonful of olive oil, to help my dogs get a shiny coat!