What if this version of Ben is another alternate dimension counterpart whos basically him but a girl and both Ben and Rook go to her dimension accidentally. Ben sees that the dimension hes in has a Mr smoothie and goes inside it with Rook and orders a smoothie while waiting at the table. After the waitress (basically girl Ben) comes over and gives them the smoothies Ben looks up and sees that shes a female version of him and that she has an omnitrix. They both see each other and then explain their situation and find out that this "Ben" works part time at Mr smoothies and fights crime. So they fight together against some other dimensional version of a villian from ben's universe.
Also, maybe Rook makes a comment on how Ben's female counterpart is "quite the fetching gal" and Ben, annoyed, replies with "cut it out Rook, thats like your saying that to me".
Also her voice could probably sound like twilight sparkle's from my little pony or Lucy's from omniverse. Basically she'd be voiced by Tara Strong.
fem!Ben: *sighs* Welcome to Mr. Smoothy. Can I take your order Kevin: *snickers before laughing like crazy* seriously, Tennyson? The minute you turn female you start working at Mr. Smoothy of all places? And that outfit! *laughs like crazy again* fem!Ben: Please, Kevin. This is embarrassing enough