Whey protein is actually VERY easily digesting and often given to people with digestive disorders. Human breast milk is 80% whey protein.
Just make sure to use a pure whey isolate. A whey protein isolate will be free of lactose. A lot of adult humans are sensitive to lactose which can cause digestive issues.
For a whey protein - you want to see nothing but that in the ingredients list. Whey protein isolate, natural flavours, a natural calorie free sweetener like stevia and then maybe some salt, soy lechitin and cocoa powder. There are a lot of good products on the market. I personally use wheytobuildmuscle from n2bm.com