Let me see if I understand this:
The cost of a 2-person room is the same for 1-person. There is no price change there. Your former friend can take his name off, but he won't get a refund. You can still board the ship.
I just hope there isn't a problem between you where they don't want you to go on either or want money back from you. If there okay to let bygones be bygones, then you should be fine to enjoy your vacation.
This is also why you always should take your wife on a cruise and not your girlfriend. People propose to their girlfriends on the second day of their cruise without confirming that she would actually say "yes" before asking her. When she says "no" and want to break up, that's an awful situation to be in with that single bed.