If your other cat recently passed, this cat is probably missing her friend. She's probably extremely depressed and does not want to eat or drink because of the anxiety and depression she feels over the loss of her friend. Do not force feed a cat. You will only choke it to death. To get her eating again, you might, for a short time, try wet food. Just to entice her to eat. Once she eats, she'll naturally drink. Once she's eating again, you can switch her back to dry food. If you try the wet food and she still refuses to eat, and it's been several days, you need to take her to the vet to make sure she's not ill and that it really is most likely depression that's causing her problem. The vet will have some suggestions on how to help the cat cope with her loss. I know you're concerned about her, but do not force feed her dry or wet food. It will most definitely do more harm than good.
In reply to your added comments, it seems you're more concerned over illness than her missing the other cat. So perhaps it's been a while since the other cat passed on and that is no longer an issue. You should never force feed a cat or any animal. I doubt seriously they would force feed it at the vet either. They would feed it via i.v. fluids and nutrients. They would not force feed it through the mouth. I know the cat is not used to wet food, but to get a cat to eat (who does not want to eat), wet food usually works. Especially if you warm it up slightly in the microwave which gets it smelling even stronger and so it will better entice the cat to eat (just make sure it's cooled before you give to the cat). Once it eats, it will need to drink. Try the wet food before anything else. And then go to the vet and see if they will set up a payment plan of some sort. But again, do not force feed your cat. You will end up hurting if not killing your cat.