[TX] Employee has extensive record or prolonged breaks and lack of consistent work. Now claims IBS is to blame.

I have an employee that has been written up multiple times and is currently on his last written warning before termination for prolonged absences from his desk for "bathroom breaks" employee has been here almost a year and this has been an issue almost tthe entire time. He now in his latest meeting has claimed that he has IBS and that is why he's gone for so long. Can we ask a company ask for documentation of this diagnosis?? How do we go about this? TL;DR: employee claims IBS disorder after a year of write ups for long bathroom breaks. Can I ask for documentation of the diagnosis? I have an employee that has been written up multiple times and is currently on his last written warning before termination for prolonged absences from his desk for "bathroom breaks" employee has been here almost a year and this has been an issue almost tthe entire time. He now in his latest meeting has claimed that he has IBS and that is why he's gone for so long. Can we ask a company ask for documentation of this diagnosis?? How do we go about this? TL;DR: employee claims IBS disorder after a year of write ups for long bathroom breaks. Can I ask for documentation of the diagnosis?