So he showed them an article about allowing musicians to pay for a ticket for their instruments... did he try to buy one? Was there not a ticket available? Did he show them this article and say that he should be able to carry on his instrument? Either buy a ticket for your $10K instrument or check it. A guitar is too large to use as a carry on bag.
Also if your instrument is worth so much and you're travelling then be responsible and get a travel case not some cardboard case.