As already said, the legal position is quite clear. The card is in his name & he is the only person liable for the debt. The cc co. will not be interested in anything else.
His only remedy - if the boss does stop paying - is a Court case against the boss or - if it is a limited company - the company. He should succeed with this & if he did then he ought to be able to make a claim against any assets the boss or company have. For example, if the boss owns a house with equity then he could possibly get a charging order on it.
However, this is not going to help him with paying the cc co. (assuming the boss stops paying), because it the time scale will be wrong. He might even have to make himself insolvent (Debt Relief Order or bankruptcy) to get rid of this debt - assuming he has no assets such as a house.