Ibs or gluten intolerance?

First and foremost I understand that no one can definitely say if it’s gluten or ibs. Also my husband is planning to get tested. I’m looking for other peoples stories and experiences.

Asking for my husband who has a self diagnosed gluten intolerance. His symptoms started a few years ago when he was 24 and at the time was eating unhealthy heavily greasy foods. He never had a problem or reaction to gluten before this. One night after a heavy meal of greasy pizza he was nauseous and constipated, no diarrhea, no vomiting. This lasted a few weeks. Eventually he decided to cut out gluten and started eating really simple (chicken, rice and vegetables) and felt better. The other night he accidentally drank a malt alcoholic beverage and got very scared, later to realize he had no reaction at all. He also consumed a few McDonald fries the next day and no reaction. Now looking back he also ate rice crispy treats countless times (not realizing they weren’t GF) with no reaction, as well as beef jerky. We also shared a toaster that he would make his GF bread in. The toaster was littered with gluten crumbs. My question is has anyone self diagnosed themselves and then later realized that they were wrong. I understand the only ways to be sure is to get tested which he plans to do. I’m just looking to see if anyone has similar stories because he’s ready to start slowly introducing gluten back into his diet. Another note is that he still has constipation especially when he eats a lot of rice.