Brief Answer:
You never know if its bacterial or viral infection.

Detailed Answer:
Hello!

Thank you for the query.

Can you please specify what kind of symptoms has he had? Was it diarrhea and vomiting? Has he had a fever? Any blood or mucus in the stool? Any abdominal pain, if yes was it cramping? Where was the pain located?
Is he better now? What symptoms does he have right now?

Flagyl contains Metronidazole. This is an antibiotic which takes action in case of anaerobic bacteria. It does not affect most of aerobic bacteria.
I assume that what you call colitis is a gastrointestinal infection. Please note that great majority of such infections are caused by viruses. Only some part of them are due to bacteria. Flagyl does not help in case of viral infection.

Viral gastrointestinal infection is usually self limiting condition. It means that after a day or two it diseapears by itself. This is possible thanks to our immune system.
Bacterial infection is harder to treat and usually needs antibiotic treatment.

So answering your questions:
1.he should stop taking Flagyl as advised
2.his symptoms of confusion and short term memory loss are most likely caused by dehydration - Flagyl does not cause such side effects
3. This symptoms will go away if he will start drinking proper amount of liquids
4. ER doctor could not tell anything about his infection. Stool culture test and stool test can answer this question
5. Flagyl was not the good choice so there is no connection between its usage and possible infection recurrence.

In my opinion he should:
1. If the symptoms are still present, stool test and stool culture test should be done. If positive, proper antibiotics should be prescribed (like Ciprofloxacine with Flagyl or Rifaximine)
2. Start probiotic intake, drink plenty of liquids and start with soft diet.
3. Next time do not just use Flagyl as you never know what kind infection he has.

Hope this will help. Feel free to ask further questions.
Regards.

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