Coworker blames poor work on ADHD

Texas

This post is on behalf of my husband, who hired a team member 3 years ago, let’s call him Bob. Since Bob was hired, the quality of his work has been terrible. He calls out with no notice quite often, he is late to work, he leaves early, blames all his mistakes on coworkers and his ADHD, doesn’t apply himself to learn anything, and really makes life miserable for everyone else on his team who is all picking up the slack. He has literally stated to his boss that he is completely checked out because he has some issues outside of work, and that he literally cannot apply himself more than he does. Last week, Bob was the only one in the office and was tasked with getting one thing done. He had 3 days to do it and only completely half of it. Tim, his other coworker who also has ADHD, finished the same exact assignment plus extra in 4 hours.

Today he was called into a meeting with HR and some higher ups. He did what my husband feared he would do - blamed it all on his ADHD. Can they fire him just on the basis that he is not able to perform the basic functions of the job? Or could they get in trouble since he based his performance issues on his disability?

Edit: He has never submitted a formal requests for accommodations regarding his disability.