I have had many cats over the years, and have seen them grieve when their companion cat passes on. For Thor, who had been with his cat for years, when she passed he grieved - his way was to cling to us humans and try to love on the poor dog who was terrified of him. Though my son and I said we would not - a short month later we got Thor another cat. They were inseparable until he passed away this May. And though Zoe loved the, now 4 dogs, and got along with the other cat she has mourned and missed Thor.
For Zoe the solution has been to give her much, much, more attention than she previously required. We would literally pick her up and just carry her around the house. We made sure to pet her whenever we walked by her, etc. She honestly got just downright weird for awhile - she would walk the house and cry, climb to new and precarious places, hide, all without a pattern.
It has been 6 months now and she has finally calmed down and settled back in. She hangs out with the dogs more than the other cats, and is usually with either my son or I more than she used to be. But, she is back to her "normal' self.
It takes time for pets, just like us humans, to adjust to the loss of a loved one. For your kitty, there is no straight forward method to help her. Keep doing what you are doing, try and encourage her to eat and drink - even if you have to bribe her with treats to peak her appetite. In worse case scenarios, you could check with your vet and try kitty anti-depressants (yes, they make those). Keep giving her love - and like Thor, she may adjust well to another cat. Sometimes they just are lonely even though we humans love them so much.
Go kiss her little nose from me.