If you google nhs ulcerative colitis you’ll find some information there about diet during a flare up. Basically it’s a low residue diet..white bread, cornflakes, white rice, etc.
She really should be able to speak to her GP, it’s not necessary to see him face to face, and perhaps ask for a referral to the gastro team, they usually have an IBD (inflammatory bowel disease) nurse there, at the end of the phone, who will be able to help. The GP should have that telephone number.