Debated in my head if to reply or not, deep breath, here goes. I think there is a good chance the employee is neurodiverse as wearing headphones that are not normally permitted. Speaking as an autistic myself, its impossible to sort criticism of what doing - in this case seaweed display - from personal attack, particularly at work. I'm 50+ and still can't until I've talked to someone ( usually DH ) who explains not personal and why. Even then I can dwell on things for months, even years afterwards. As a neurodiverse, there's always the fear of losing the job over "doing something wrong" which is something others wouldn't bother about. For example, I rung up something wrongly on card machine but correctly on till so till was short at the end of the day. I offered to put the difference in ( a small amount ) and manager says "we're human, mistakes happen, don't worry about it".
The employee in question may have gone down that thought process and fear came out as anger. Yes, wrong to do so but we have trouble with emotions especially when in sensory overload - I do and start using the f word and hitting things, not people or animals I hasten to add. Its not that I can control it, won't control it, Its I CANNOT control it. I feel terrible afterwards too.
The cashier you spoke too may also not understand neurodiversity/ autism, or worse, believe its not real and/or adults don't have it, like we grow out of it something
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Was the employee in question young? Taken me decades to learn what not to say, but I still come out with the inappropriate things - eg NOT say "Wow, you've put on weight" but say "You look different from last time I saw you"
Conclusion? As others have said, use it as a teaching moment to explain neurodiversity and move on.