Hello, I'm Dr. Deb.
I recently came online and see that your question about Trinity hasn't been answered. I'm so sorry that you've had to wait for a response but I'd like to help if you still need assistance.
I wouldn't expect gastrointestinal signs of this nature to be secondary to an infected tooth although I can understand why you wife might think they're related.
Cats who are ill or stressed often experience a change in their fur, especially if they're not grooming as much so I might not worry too much about this just yet.
In my experience when a cat this age has these symptoms, the possible causes are either secondary to a systemic disease (issues with kidneys, pancreas, liver, for example, OR possibly secondary to Late Onset Inflammatory Disease or possibly something which she might have ingested (such as otc or prescription medication). There are other possible explanations as well (such as viruses or bacterial infections) but these seem less likely since she's an indoor cat.
Regardless, I do have a few suggestions which, hopefully, will make her feel better:
1. Over the counter Pepcid AC can be given but I tend to suggest a slightly lower than normal dose if I'm uncertain as to the kidney or liver status of my patient. Therefore, I'd suggest 1/4th of a 10 mg tablet once a day.
This drug may help to reduce any nausea she's feeling.
2. As long as she's actively vomiting, feeding her will be counterproductive since it's all going to come back up. If she seems less nauseous, then I'd offer her boiled chicken breast (with or without the skin is fine) or human baby food (avoid those with onion or garlic in them), but only a small amount at one time.
3. It's fairly easy for cats this age to become dehydrated which can exacerbate the situation. I'd offer her human Pedialyte although she may not willingly drink it. If that's the case, then I'd dribble small amounts into the side of her mouth where there's a gap every several hours. The fluid requirement for healthy cats is about 1 ounce per pound of body weight in a 24 hour period to give you an idea of how much to give her.
4. There are no safe over the counter anti-diarrhea medications which I'd suggest since cats can be sensitive to many of these products. However, feline-specific probiotics may help if the stools continue to be loose once (if?) the vomiting has subsided.
Forti Flora is a good one which may be available at local pet/grain stores or online; it's a powder which can be sprinkled on the food once a day.
5. If the vomiting has subsided but her stools continue to be loose, I'd continue the boiled chicken breast. It may not be nutritious over the long term but several days of eating it should be fine.
I do tend to worry when cats this age develop gi symptoms such as this since they can deteriorate fairly quickly. But, I hope this provides several options for you to consider and that she'll respond.
Again, my apologies for the delayed reply. Deb