It’s a common problem in diagnostics and it’s why mammograms aren’t recommended to women under 40.
Let’s say you have 10,000 patients. 10 have cancer or a precancerous lesion. Your test may be able to identify all 10 of those patients. However, if it has a false positive rate of 5% that’s around 500 patients who will now get biopsies and potentially surgery that they don’t actually need. Those follow up procedures carry their own risks and harms for those 500 patients. In total, that harm may outweigh the benefit of an earlier diagnosis in those 10 patients who have cancer.
Personally I’m not sure the gate keeping you’re observing is all that much of an issue. I think it’s useful to remember many vegans are also public advocates for veganism. It’s important to them that people generally know what they mean when they advocate for veganism.
However, the definition of all words are always in flux. It’s not uncommon to see people call themselves vegan when a more apt description of their lifestyle would be plant based, flexitarian, vegetarian, etc. As such, I think edge cases like your friend take on an outsized importance that goes beyond the morality of your friend eating honey.
Basically, the goal may not be the social exclusion of your friend which is what I think is usually the problematic aspect of gatekeeping.