Just so you know, it's discriminatory and illegal in the United States to fire people because of health problems! Not to mention, it's a cruel thing to do! The boss in this "Am I the A-hole?" post doesn't seem to understand that.
OP explains that he's been policing his employee's bathroom breaks. He's noticed that she sometimes goes "as frequently as thrice per hour" and that she takes "medical leaves with constant stomachaches and diarrhea." He says that this impacts her performance in her job because her absences means she struggles to keep up with the team.
That's a legitimate concern to have, and it's a boss's job to make sure that his employees have everything they need to do their jobs effectively. If she misses a lot of work due to her health condition, there are ways to make sure she is supported and keeps up with what she has to keep up with.
Instead, her boss wants to fire her. Part of his reasoning is that he sometimes noticing that she eats "Indian and Chinese food with heavy spices and lots of chili peppers that the smell alone can make one's eyes water." Based on no information about how her body works, he thinks that she should be eating "bland food for weak stomachs."
He actually brought up his employee's diet with her, which is extraordinarily inappropriate. He suggested that she avoid spicy foods during the work days, but understandably, she balked at this suggestion. It is, after all, her own life.
OP doesn't understand that, though. He gave her one month to "improve," and if she doesn't, he wants to fire her. But he can't in good conscience give her a deadline to get her health problems in check. That's not how medical issues work.
"YTA," one commenter wrote. "Most digestive issues vary person to person as far as triggers. A somebody with digestive issues, I'll tell you what bothers me does not bother other people."
Perhaps she can tolerate spicy food but gluten or dairy gives her digestive issues. Maybe her issues are more complicated and unpredictable than that. It's not his responsibility and it's not his problem. And it's certainly not his right to try to get involved in her personal life and medical issues.
"I love this white insistence that her 'exotic' diet of Indian and Chinese food is what's making her sick," another person pointed out. "Indian spice mixes contain several spices such as licorice root, turmeric, and ginger that aid digestion, and Chinese food is also heavy on ginger and anise. Spice doesn't mean an upset stomach. How would 70 percent of the world function otherwise?" A very good point.
Another commenter suggested that OP may have opened his company to "liability issues" by even suggesting that he wants to fire her because of her food choices and medical problems.
But other people understand OP's position. It is hard to have an employee that's not working to your standards. "Well, I would just ignore the bathroom stuff in your place and just focus on numbers," one person wrote. "If she is still badly underperforming then fire her on these grounds and nothing else."
The bottom line is that OP has not handled this situation well from the beginning. There was a way to address her poor work performance while making sure she was supported and had everything she needed to be successful in her position even with her health issues. OP dropped the ball here.