I find this claim perfectly credible. She vomited immediately, continued vomiting, was ill afterwards (typical symptoms), and the symptoms resolved, as would be expected with a small dose. The body has an enzyme that converts methemoglobin back into normal hemoglobin. It's just very slow in comparison to getting treated with methylene blue.
Just FYI, you can vomit out stuff that has already passed through the stomach into the duodenum. Any number of google searches will confirm this. For example, here is the top search result for a search on "can you vomit from the small intestine?" It says, "Vomiting is the forceful expulsion of contents of the stomach
and often, the proximal small intestine."
@Belle1990, I believe you.