Recently we were assigned a big contract with a big company, who manufactures headphones. We have a developer which works on this project, who always works with his headphones on. Normally, this wouldn't be a big deal at all, as it's perfectly normal for our employees to work with headphones on in our company.
However, I perceived a problem when he got assigned to this project, because his headphones are made by a direct competitor of our client. We told him not to use those headphones because he is working on this big project (worth 60% of the company revenue) and he responded by saying he is not prohibited in any way from using headphones during work. HR also told us that when he was hired he explicitly asked to work with this brand of headphones on and no other (don't know why, seemed strange, but perfectly acceptable by HR at the moment).
The situation escalated quickly when we were informed that at anytime, an emissary of the client may come to check on the work, so it is perfectly reasonable to assume he will get discovered.
Since he has every right to wear his headphones, and HR signed him on explicitly granting these rights, how we can handle the situation?