I am so sorry for your loss. 
Our boy grieved terribly when we lost Oldgirl last spring. He was depressed, just lay in his bed all day, refused to go in the garden alone and started howling when left - even for just a 10 minute school run, as well as at night.
It's a complex mix of missing their friend and never having learned to skills involved with being on their own, which leads to anxiety.
I upped his walks and spent lots of time outside in the garden with him. The only thing that seemed to bring him out of himself was when we bought a fling and fetch and did lots of ball throwing so he could just run and run, but he'd go back to being sad and depressed as soon as we were back in the house. At one point I was walking him for four hours a day, plus ball throwing in the garden and clicker training sessions trying to both distract and wear him out.
We knew that we wanted another dog, but I wanted him to learn the skills he needs for being alone first, so did a separation anxiety programme with him before we brought another pup. We lost Oldgirl on 25th April and brought home Pip on 9th August, by which time Lurcherboy was more settled and able to cope with an hour home alone. He wasn't impressed with the pup at first and still isn't attached to him in the same was as he was our old girls, but he's definitely happier in himself and doesn't have any SA symptoms anymore, so it definitely helped, but wasn't the 'cure', iyswim.
Things worth trying are Pet Remedy or DAP and possibly a thundershirt to help with the anxiety. (Thundershirts have a 100% money back guarantee if they don't work and Pets at Home honour this, so it's worth a try, as it won't cost anything if it doesn't work for your poor girl.)
It's very early days for both you and her and she's bound to be picking up on your grief, as well as feeling her own sense of loss and grief. Hopefully she'll gradually become used to life without her friend, but it will take time.
