Yes, if in proper amounts and well cooked.
Everyone has their own opinion, you just have to go with what you think is best.
Our vet has the attitude that dogs should eat a tiny bit of everything, so that they don't get 'delicate stomach. Of course i am not talking about cookies and candy etc..! I am referring to any natural found product.
I make/cook my dog's food and every couple of days give a meal of a very good quality dog food like Orijen (amazing what you can learn by reading the ingredients!). The reason for the prepared dog food meal is to keep her used it for when we are traveling or away from home when she cannot have her homemade meals.
Each breed has its own needs, and it also depends on the activity level. Ours is a miniature pinscher that doesn't have an 'off'' button! so her diet consists of 75 percent protein from meat sources (rabbit beef, pork,lamb), 15 percent from legumes and grains (lentils, garbanzos, beans, bulgar wheat, oats) and 10 percent vegetables (everything EXCEPT mushrooms, onions, and garlic which are not very good for a dog)
Sorry for the long winded answer!
So yes, your dog can easily eat lentils as long as they are well cooked (all veggie and grains/legumes must be well cooked as they won't receive the vitamins and minerals from them if not), introduced slowly, and never consisting of more than 10 percent of the meal. Just like humans they will get a tummy ache and bad wind and poo if they eat too much of them. (i myself adore lentils!)
At least that is what i have been told :)