Are you dealing with a travel agent, directly with the cruise line, or what?
If I was the potential roommate I would also feel better paying for the cruise to the cruise line or agent, rather than writing you a check - only because it is more of a "sure thing".
But I don;t know why a "broker" wouldn't make a change like this for you, it must happen all the time.
By the same token - you could call the cruise line and tell them you now want to bring a guest and request a booking number. (I am assuming you already paid full fare for two people". Whatever outstanding charges exist (port fees) they cruise line should be willing to take directly.
Is it possible you went with a broker who is asking for a change fee to add this person? if so - just add it in to the cost of the cruise.