First of all, I hope your husband's grandmother is feeling better.
Now, let's see if we can get that diarrhea stain out of your carpet.
I have written some step by step instructions for diarrhea stain removal here, which may be of use. It's part of the A to Z Stain Removal Guide I've got here on the site, and discusses general tips for removing stains from clothes, carpet and upholstery.
Now, for some "TMI," also known as "too much information" about removing diarrhea stains.
The stain removal technique for getting out the stain depends, to a certain extent, on what was in the diarrhea. Yuck, I know.
So, I doubt you will be able to pinpoint with certainty what "made" the diarrhea, so instead we make some assumptions, and try a couple of different techniques, one at a time, for stain removal to get rid of the major types of "body stains."
Looking at your list of what you've tried, I don't see that you have
tried hydrogen peroxide or ammonia. Both of these would be at the top of my list of stain removers to try, and both would be applied with an eye dropper.
If you'd like a homemade carpet stain remover recipe, I've got one using just 3 ingredients! It features hydrogen peroxide, along with two other ingredients, that can remove lots of stains (including vomit, so I think it would also work well in this situation). Here's the carpet stain remover recipe link.
Remember that before applying to the stained area apply a small amount in an inconspicuous area to make sure the stain removers will not harm your carpet.
(For example, if you have a wool carpet DO NOT use ammonia, because ammonia dissolves wool.)
You may also be seeing the residue of a protein stain, and an enzymatic detergent may help remove this stain, but it will really foam in your carpet and is likely to attract dirt if not thoroughly rinsed out, so keep that in mind.
You can check out carpet stain remover reviews here, if you'd like to try one. The ones designed for pets, and those which also deal with any lingering odors at the same time may be the most helpful in this situation.
I hope you enjoy this gift, and stop by again soon!
CAUTION: This website is provided for informational purposes only. It is provided as is, without warranties or guarantees. Some stains and messes just won't come out, and are permanent. Further, some cleaning methods can harm your item, so if what you want to clean or launder is sentimental or expensive call a professional. See disclaimer of liability for more information.