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.
"We give dogs the time we can spare, the space we can spare and the love we can spare. And in return, dogs give us their all. It's the best deal man has ever made" - M. Facklam
"We are raised to honor all the wrong explorers and discoverers - thieves planting flags, murderers carrying crosses. Let us at last praise the colonizers of dreams."- P.S. Beagle
"All that is gold does not glitter, Not all those who wander are lost; The old that is strong does not wither, Deep roots are not reached by the frost. From the ashes a fire shall be woken, A light from the shadows shall spring; Renewed shall be blade that was broken, The crownless again shall be king." - J.R.R. Tolkien