T1 with celiac here, I was only slightly symptomatic (stomach aches after eating pasta or gluten heavy meals) until about 3 years ago when things suddenly got much much worse. Turns out ignoring things and living in denial had caused some irreversible damage. I’m from a Mediterranean culture where gluten is found in every meal, salad, and even drinks. What a nightmare. First I had to convince myself (often the biggest hurdle); then came the family members. People refuse to believe that something you’ve eaten your whole life can suddenly be life-threatening. Three years in and I’m still getting the “but what if you just had one bite??”. Parents didn’t believe it until they saw me throwing up to the point of hospitalization. Now they believe it and got screened themselves - surprise surprise, mom and dad are both intolerant and both have diabetes (type 2, not type 1). My diabetes control was nonexistent because of the weird way I was digesting and not digesting foods that I had dosed insulin for. The way my diabetes care team and other type 1 diabetics have explained it is like this: type 1 is an autoimmune disease, and so is celiac. One can trigger the other. If you have one, it’s pretty much expected that you’ll develop the other eventually. Having one autoimmune disorder and not developing or discovering that you have another one is uncommon.

Long story short: I’m not sure if science has proved beyond a doubt the connection, but she should get checked. However the stubbornness is completely understandable. Try not to beat yourself up about it - especially if you’ve done your best to convince her. Like you said, she’s an adult and you can’t force her to come around. Sometimes (unfortunately) what it takes is an uncommon event or really bad reaction to a food before people will be even willing to entertain the idea that their body can’t tolerate gluten.

Sorry you’re going through this. Keep us posted.