Weena is a guide only if you squint really hard. While she lets the Time Traveller know that fear still exists in the world, it's possible that she knows even less about her world than he does. She does accompany him everywhere, and she provides a sense of home and companionship. But she's really not that great of a guide – and the Time Traveller really needs one. He even comments on the fact that in fiction he's read about time travel, the stranger always finds a native guide to explain things. He, on the other hand, only has someone to give him flowers. (Still, we'll take flowers over nothing any day.)
Besides being a poor guide, Weena's main role is as a sympathetic companion for the Time Traveller. When he realizes that the Morlocks eat the Eloi, the Time Traveller feels bad for them largely because he's spent so much time with Weena, who has come to seem human to him. As he notes, "She always seemed to me [...] more human than she was, perhaps because her affection was so human" (8.2).