Having one budgie alone is fine, if you hand tame her and give her lots of human interaction. That way, you essentially become her "mate", and she is happy. It is best to do this with a young budgie, and they become "set in their ways" as they get older, and more difficult to hand tame. It's certainly not impossible, but more difficult, as they get older. If you intend to keep her alone, you really, really need to take the time and effort to hand tame her, even if it takes a year.

If you don't have the patience or time for that (I don't!), then you need to get her another budgie for company. Birds are not solitary animals, most live in flocks in the wild, and have evolved to need company. Keeping one bird alone is much like being in solitary confinement to the bird.