You should take her to see a gastroenterologist, or at least get a second opinion. Bleeding with your stools is not a symptom of IBS and usually indicates something else like Crohn's, ulcerative colitis, etc. So does weight loss.
IBS is a diagnosis of exclusion: meaning that you must be tested to rule out all other identifiable physical causes such as Crohn's before you settle on an IBS diagnosis. Without having had a colonoscopy, endoscopy, blood tests, etc you cannot assume IBS.
I would start by demanding a colonoscopy. I don't suffer from Crohn's. I have had IBS for about 9 years now and have been through all the testing to rule out other causes for my symptoms. The first place the doctors started was with a colonoscopy. It's no walk in the park, but it will provide the best picture of what is going on in your mother's GI tract.