I don't know the law, but it may be legal in some states, and as such he should be able to disable it and ask about it here. If that is the case, though, a call to the dealership should answer the question.
Personally I would never get in a car and not wear my seat belt. I think it's stupid. However, I also think it's ridiculous that there are laws about an adult, in America, being forced to wear a seat belt (or a motorcycle helmet for that matter). The only person they are potentially hurting is themselves. It's different with kids and the incompetent, but for adults I think it's just wrong. I think it's a case of a government that is getting to0 involved in people's personal lives, and more so them trying to create revenue with ticketing.
That said, I still would always wear my belt. A lot of other mods are talked about here & elsewhere that are illegal for both safety and non-safety reasons and people seem more responsive. I understand why you may not want to help, but then just don't help. Though, wearing your seat belt is a solution, and it would stop the chime. That also explains some other solutions, such as clipping the belt in and sitting on it not under it, or the solution offered for using a cut seat belt clasp from another car.
All the guy wants is some suggestions as to he can disable it. Judgment should be reserved for a different forum.