Doesn't matter, you do not need a professional diagnosis to make sure someone is being rude or just plain bad.

Although, I'm ADHD myself, contrary to popular belief ADHD can be the reason for toxic behaviour sometimes too. The impulsivity and different reactions they may have to situations and so much more can create the ground for toxic relationships easily.

As a rule of thumb, you will often notice someone with ADHD doesn't do the said action with bad intentions and it's more of a fault in their hard-wiring. As i said, doesn't excuse but it does explain it. At the end of the dat this is manageable for ADHD people (if they want to) with medication, therapy and mindfulness.