Firstly let me say Karma to you for taking in a rescue budgie.
Being a rescue can make the whole process a bit harder as you have no background or information regarding her.
In regards to perhaps she was used to having another budgie around, if you can happily accept two budgies will bond to each other far more easily than a human flock member then yes get her a feathered friend.
On the other hand if you have the time and patience needed to work through her reluctance then keeping her as a lone budgie would be more beneficial to you.
My own personal view would be to get her a friend and simply go about training and befriending the two of them. The joy you will get from watching them both interact and live happily will be a huge plus in my way of thought.