@Bella Dea,
Disclaimer: I am not a physician or dietician and this is based purely on what I learned when helping a dear friend who suffered from this miserable disease.
While it is true that there is no single prescribed diet for Crohns as it takes many forms and everybody is different, there are a few tried and true wholistic remedies that seem to benefit most.
First your SIL knows to avoid alcohol, fried foods, high fat content yes? Anything that puts a lot of stress on the liver?
She should look at foods rich in sulfur and sulfur-containing compounds. Sulfured molasses as a seasoning for toast or pancakes for instance. (To gain weight my friend ate stacks of pancakes sweetened with sulfured molasses.) Other foods rich in sulfur are onions, garlic, and egg yolks. (Be careful of too many egg yolks due to their high fat content, but my friend tolerated them well.) Other foods she found benefit from included broccoli, brussel sprouts, collard greens, cabbage, and I think cauliflower. All are high in sulfur.
Some spices also helped her--they presumable detoxify the liver--turmeric and cinnamon were two. Also black licorice.
At any rate, even if they don't noticably help, I don't think any of these things could hurt.