There are a few things that I can offer that might help Ziggy.
Since the I in IBD means inflammation, the very first thing I would do for Ziggy is to give her something to soothe that inflamed gut. The best thing that I know of that is soothing and healing is Aloe. The best aloe on the market is George's Aloe. My sister told me about George's Aloe more than 20 years ago. She has a history of inflammatory gut, she was eventually diagnosed with Hashimoto's and Lupus. Anyway, George's aloe has no preservatives, is shelf-stable and no bitter taste. It tastes like water. As for how soothing it is; I suffer from GERD and when I have a particularly bad night of reflux, I get a glass of George's Aloe and sip on it. I can feel the raw pain from the acid reflux soothed with every sip. You can get George's Aloe at your health food store, Whole Foods and online. I would put George's Aloe in her water and in a small dish beside her water bowl to see if she will drink it willingly. I would also give her some via dosing syringe daily to make sure she got some in her tummy. George's Aloe doesn't cause the cramping and griping that other aloe brands do.
Read up on the reviews of George's Aloe on Amazon.
The next thing that is recommended for IBD is the amino acid glutamine. Glutamine helps heal the leaky gut. I'd follow the advice in the 2nd link and give her 1/16 tsp to start. You can buy measuring spoons 1/8th, 1/16th and 1/32nd on amazon.
Probiotics would be a good idea as well. ProBios is formulated for animals, you can get it in a powder form. You can get it online (Amazon) or your local Tractor Supply or Southern States if you have either of those near you. It is cheaper than Purina's Fortiflora and I think works better. It contains the strain Enterococcus faecium which is crucial to good gut flora.
The cause of IBD is generally unknown but major suspects are autoimmune disease, bacterial or viral infection. If there is a mycobacterium responsible for Ziggy's IBD then colloidal silver would be a good addition to her regimen. Colloidal silver effective against viruses. One study has shown CS as effective in reducing the inflammation of ibd. If you live in the States, I recommend you order from UtopiaSilver.com. They have the best price of CS. I've used their products for years myself and on my cats. I have used it for UTI's, eye infections as well as skin infections internally on my cats. I usually dose my cats 5ml 2-3 times daily when they're dealing with some kind of infection. Once you get her IBD under control, you can put 5ml in her water bowl and that would probably be all she would need. And to dispell and fears you might have about the use of CS and argyria aka 'The Blue Man', he made silver chloride, not colloidal silver. He threw salt in his water which created silver chloride and he drank 20 ounces of his silver chloride for years. Properly made colloidal silver with distilled water, electric current and nothing else will not cause argyria.
Finally, I'd consider making her food so you can control the quality as well as the ingredients. Her IBD may be exacerbated by food allergies or sensitivity as well. But I would start simply with a recipe of 80% protein, 10% veggies and 10% brown rice. LIke chicken, green beans and brown rice. Include hearts and liver too. You'll need a good vitamin supplement for her as well. She's going to need vitamins and minerals. That's gonna take research, I'm afraid. Anitra Fraizer's "Natural Cat" has a good recipe for a vitamin-mineral powder. You'll need to find a good vitamin a supplement though as cats cannot convert beta carotene to vitamin A. And I'd consider adding co-enzyme Q10 to her list of supplements for its anti-inflammatory properties and it's protective properties for the heart, liver, and kidneys.