Well, everyone's different, and with your mother's constrictions she may have more limitations than others. My mother also has Crohn's, and has had it since her 20's, off and on. Avoiding high fiber foods never helped her when she was younger, though those were the recommendations back then. She now eats a whole food diet, with abundant vegetables, high quality proteins, good fats, lots of fiber, etc. She keeps the quantities small (she weighs all of 90lbs), she has a small dessert every single day, and she's thriving.
She also takes supplements. Because Crohn's is autoimmune, she keeps her vitamin D levels above 50ng/mL. She also takes B12 and has gotten shots in the past. She takes a whole-food multi, an Omega 3, a strong probiotic, and a whole-food bone supplement (she has osteoporosis). She walks a few miles everyday and takes Yoga, Jazz dance, and aerobics. She's 80. After all those surgeries, I'm pretty certain she has multiple nutrient deficiencies. If she's not getting B12 shots, maybe have her get just one to see how it makes her feel. Are her surgeries well healed, or is she having issues with them?
A book your mother might find helpful (or you, for that matter) is Jordan Rubin's The Maker's Diet. Not everyone is crazy about its Christian orientation, but because he himself almost died from Crohn's, his story of regaining health gives his recommendations more clout. I live in a community that has a high number of very devout people, so I recommend this book quite a bit, and everyone so far has had deep appreciation for it. One of my first clients had just been diagnosed with autoimmunity. The doctors didn't know how it would manifest, but suspected rheumatoid arthritis. She followed the diet recommendations in this book and immediately felt better. It's now been 5 years, and she's healthy and has gone on to have 2 more children. She looks great. The diet is basically E4H, but it has that beautiful added touch of being a personal story.
Another suggestion is to have her remove gluten from her diet, which is highly associated with autoimmune conditions. She can try an elimination of it for a month and then reintroduce it to see if it produces any symptoms. Not everyone with autoimmunity is gluten intolerant, but it's worth checking out. Crohn's, being an inflammation of the small intestine, means she has leaky gut, so you want to remove any known allergens from her diet, maybe do some testing or elimination diets to discover them.
High quality whey protein powders are soothing and healing to the GI tissues. You can find this whey in the Bauman Vital Scoop product, and you can get it online through mercola.com, or through other nutrition consultants. Vital Scoop has loads of other nutrients in it, too, and it's pretty tasty. Smoothies are easily digestible and nutritious, so these might prove wonderful for her. There are also rice protein powders available that would be suitable if she has dairy sensitivities.
Best of luck to you.