Whenever I get or recommend a whistle for a beginner-& I often have-I usually get a Feadog, the Guiness is just fine, the basic "Little Black Whistle" is good. They are usually under or about $10, & sound pretty much like the average whistle you might hear in an average Ceoli band. And, of course, I always get it in D if the person has no experience or no preference, as most likely the Whistle will blend in with the most oft used keys for Irish, Bluegrass & other folk Music.
    When I teach workshops/classes, I specify that they only bring a "Key of D" whistle. Then I have them listen to a good whistle player who plays in the Irish style! It really matters if that is the most likely Music they are wanting to play.
    I have also found that folks from many countries have an instrument which is nearly the same, & can sit down & play it quite easily. When I'm out & about Whistling, I most always have an extra D or 2 or 3 in case someone comes along who wants to "sit in" or just try it out. Of course, I am very beginner friendly, but not all Music sessions are.