Had one removed at the doctors off my back after picking part of it off, then it becoming a problem.
the doctor said they are a bit of a faff for them as they don't get paid sufficiently for their removal to justify the time and materials that it takes to do, and should really refer to the hospital now as their surgery has a clinic but it wasn't getting sufficient throughput to justify it. I said it had been ruining shirts and needed sorting asap, and to his credit he did it a bit later the same day.
all that they used was in effect a "medical soldering iron" (to cauterise the wound) and a scalpel to chop it off, then bit of iodine solution, swab and some tape.
as a way forward, perhaps offer to see if there was a way of getting them removed, as the alternative seem like there is going to be a standoff between need to do it and unwillingness.
seems however to be a different issue as they went to the doctors to get a note, perhaps the same doctor could resolve the problem to make him compliant, or speak to occ health to go for treatment via a letter from them to the doctor.
seems to me that the individual has found a way to relinquish the role and get it given to someone else or there is potential for the hr interaction leading to dismissal and then tribunal, neither side would benefit, so a compromise is in order if desired, (which you express as an intelligent reply).
perhaps a without prejudice meeting with HR and you in attendance for a period if they refuse to address with occ health or their own doctor. note there could be an extended delay for the procedure waiting time, and you would have to have some sort of plan as to what to do in the interim, and then once that's in place. there would be questions as to why that couldn't contine longer term, so will have to get to them either doing it or not.
there isn't going to be a middle ground as the others will also look to see whats acceptable as you're going to set a precident, so it's a question of how they get the messaging in an appropriately compliant way to take further via hr, (if needed to dismissal), whilst remiding them that they could get the issue resolved by getting it chopped off if it's in an actual area where it would affect the fit of the masks, and other masks would miss them
if they don't want to work there, or don't want to be employed with your organisation, perhaps the sooner they look the better. perhaps showing them this note would allow you to remain independent, whilst suggesting the options available, and evaluating pro's and cons.
Don't know if redundancy is where they want to go or get to, or compromise agreement, however it would be easier for all concerned if they get back to where you are all in it working for the same objectives.
someone in an organisation mentioned that they had employees they would be sad to loose, and some that they would be less so. depending on the categorisation, it may be a full and frank conversation that convinces all parties of the best outcome to the situation.