I don't have IBS, but I do have IBD, Crohn's/Ulcerative Colitis to be specific. Until I got the right medication, I had similar issues to the ones described. There were a few things I couldn't eat because they'd start a flare, but uncooked apples were the worst. I hadn't eaten any raw apples in over 10 years whe I couldn't resist the fresh apples with honey dessert I ate at a restaurant in Greece. I was terrified by the potential reaction I'd have, but nothing happened.
I have a cousin who had such severe IBS that she had to stop working. Many years later, she finally saw a real gastroenterologist. That doc put her on some kind of immodium regimen and it's been like a new life for her. She didn’t want to take any meds for it. I totally understand that, but sometimes they are necessary.
I don't know the differences between the 2 afflictions and I don't know how their treatments vary. I do know that there's no substitute for proper experts and testing, like colonoscopies. And I always take Pepto with me on my trips just in case I get exposed to something that can cause symptoms.
Toilets are always on my radar, no matter where I am. At my age (78) there are 2 reasons for that. ;-) As for toilets in Great Britain especially, I always try to use the handicapped ones. That's mostly because of my knees now, but it started because they were often the closest to the main floor in the public buildings I was in. I appreciated the signs on the doors of some that said, "not all handicaps are obvious."
Fortunately, your trip is far enough in the future that your family member has time to see a gastroenterologist and pursue some possible options to make their daily life better so that they can travel and do other activities without such worry. My cousin was literally trapped in her house and afraid to leave for years!
If your family member has already seen a gastroenterologist and been told there's nothing to be done, I'd find a different gastroenterologist.
By the way, I was first officially diagnosed with Crohn's when I was about 34 years old, so I've been dealing with these gastro challenges for about 44 years. I've had 2 surgeries that removed about 2 feet total of my colon. Over the years I've taken 4 different medications, two that didn't work at all (a steroid and an immunosuppressant) and one that worked well for the first 24 years (a sulfa based med). I finally got the best medication for me about 20 years ago and I'm still taking it. Of course, it's the most expensive of the lot. ;-) It definitely was a long and winding road, but I was determined to not let my gastro issues completely control my life.
Except just before I was first diagnosed by a gastroenterologist (I'd lost about 35 pounds, had a constant low-grade fever and couldn't eat much of anything), I was never as sick as you have described. Getting that first prescription that worked made a huge difference.
I hope your family member can find a good gastroenterologist with some acceptable solution for her gut problems, the sooner the better.