Storygaming stuff aside, there ARE party "roles" in a dungeoncrawler. The Fighter, Thief, Cleric, and Wizard are obvious examples- they have particular strengths along with areas in which they're lacking. Together they complement each others' abilities. The Druid, to me, is like the Bard: she's less about filling a "role" and more about bringing a certain style to the game.
If you've got a Druid in the mix, you know you'll be seeing a lot of man vs nature conflict, for example. I would tell your player not to worry about their "role" too much. Combat doesn't work that way in DW, it's not about having a laundry list of abilities. It's much more versatile.
Instead, tell them to pick the Druid if they're interested in the class's style and what they can bring to the campaign. Play a Druid if you want to see one at the table, you know?