I am surprised that she wasn't started on antibiotics when diverticulitis was diagnosed. There may be a reason as to why she wasn't, but to me it's rare than someone suffering with diverticulitis isn't on them.
It does sound as though further investigation is needed though. There are several investigations that can be performed, such as a full colonoscopy, CT abdomen, CT Pneumocolon, barium enema, and more. However, the right invesitgation required is determined by the symptoms she choses to share with the medical profession. If she chooses not to tell her GP about what she is experiencing, them the cause of her symptoms can't be determined. I'm not intending to lecture as I can understand why she doesn't want to tell all to someone else, but she really does need to be open and honest about it.
I have seen many people who have chosen not to have their symptoms investigated earlier because they were too embarrased. Sadly a lot of these have gone on to develop further complications.
I can understand the worry about the 'C' word, but if and should it turn out to be, then it can potentially be cured very easily. However, if left too long, then the potential to spread and develop further increases significantly.
If she does have any episodes of bleeding then I would strongly recommend that she goes in to hospital and be admitted for investigation. Yeah, it's not nice, but it may need to be done. If the symptoms are continuing regardless, then she needs to go to her GP and be honest with them. They should make the necessary referrals to be investigated.
Sorry for the essay, and I'm not intending to lecture. I hope that it is something relatively simple and can be sorted very quickly.