Hair follicle drug testing

I would argue you are both partially right. It might be possible to detect a single use for a non chronic user after 90 days but would unlikely meet the requirements for a positive result.

Hair has a major problem that blood and urine samples don't have which is contamination. The hair follicle has to be treated by a medical/chemical professional and washed in a specific way so as to remove any external contamination without destroying or otherwise compromising the sample. As such there are sample preparation requirements and concentration thresholds post-wash that a sample must meet in order to be considered a positive sample. What those thresholds are would likely change depending on the country and the nature of the job, someone working with a drug crimes unit or in evidence is likely to have much higher occupational exposure (or even accidental ingestion) than say someone working in the IT department. There are different biomarkers of cocaine use as well such as benzolecgonine, cocaethylene, and norcocaine that can all be looked for and can be used to differentiate between problem users and people with minor exposures.

After 90 days of a single use I suspect your sample's concentrations, while still potentially detectable with good equipment and good samples, would fall below all but the most stringent reportable limits, but don't quote me on that if you end up losing your job.

Hair testing is much more involved than peeing in a cup, I can't imagine it will be rolled out across the board unless the departments suspect major abuse of the stuff by the staff.

The best advice is just don't do cocaine if your livelihood depends on you passing a hair test. If you're told you have a medical in 90 days and you can't get clean for that medical then you have a problem in my opinion.

References:

https://www.sciencedirect.com/science/article/abs/pii/S037907389602052X

https://www.sciencedirect.com/science/article/abs/pii/S0379073804006528

https://www.sciencedirect.com/science/article/abs/pii/S0379073814002308