There have been comments on this board about people admitting to being in contact with an infected individual, and not one of them were denied boarding when providing a negative test taken within the time frame required by the cruise line. They were, though, required to go through a special screening with the ship medical staff. Most reported just having the medical representative doing a visual examination. I assume that if you are exhibiting obvious symptoms, it might be another story.
Now, admittedly, these were anecdotal stories on these boards from folks of which I don't have personal knowledge, but it does jive with Carnival's posted protocols. But, in the end, it is up to the decision of the medical staff.
CDC still, as of today, defines contact as being within six feet of an individual for at least fifteen minutes. Just in case your husband was sitting across a large table from this "friend" for most of their time together.