I don't know if she really needs or should hire a stylist. Nina doesn't even attend that many events and most of them are not really fashion related (e.g. MuchMusic video awards, CW upfronts). She's not attending fashion events (fashion shows, galas, dinners) where she has to impress fashion editors and the like. Obviously she has to look good but I think hiring a stylist would be a waste for events like these because they're not particularly important fashion-wise. Good stylists cost $$$.
But when her career starts to get hot and she's attending dozens of events and doing movie promo tours, she should get a stylist because then she'd really need one since there's a lot more attention (see Mila Kunis and Olivia Wilde). Otherwise she's going to turn into Camilla Belle, who looks fantastic all the time but gets mocked for having a red carpet career.