I did just that on my upcoming 8 night Carnival cruise.
The 200% single supplement made the room cost about the same for just one passenger as for two passengers. When I later added my friend, he then paid for travel insurance, pre-paid for tips, daily fuel surcharge, and the cost for port fees and taxes. His total above and beyond what I had previously paid came to approx $300.
I predict your cruise line will be quite similar.
Now comes the part, do we now split the entire bill in half, or does he just pay the $300 extra................