I expect she will have grit in her crop from before anyway, and this is only a temporary feeding method. But at this stage if she is so sick that she cannot eat by herself and you have to tube feed her, then I think grit is the least of your concerns. She will either get better and eat for herself or she will deteriorate and die.
You can either nurse her and see what happens, or euthanize her.
Sorry to be so stark, but it is what it is. I do hope you manage to turn her around. If she ingests 15ml x 4 per day and it is a full crop full, then I'd up it to 6 times a day. You are already giving her vitamins in her water aren't you? I don't remember.
Best of luck.