EMG, your a good friend.
As far as I know, these fish are much less hardy when that small. However, as a whole, they are very hardy fish. The fish that I had supposedly spent a few hours in a fresh water tank and in the process, all his fins rotted off and he developed tons of soars. The LFS owner was able to get him back in time and treat him. He came back better than new and was happy as a clam when I had him.
The swimming face down thing does not sound bad. Mine, though much larger, would hide behind the rocks face down, would swim face down a good bit, and would even lay on his side in the corner occasionally. Nothing was ever wrong with him. I just think they are smarter than most people realize.
He will definitely outgrow the 55. How soon is another question.
I would say this is the biggest problem with these fish. Most people (including myself) get these fish and are not prepared for how large they will get and how long they will live. Its almost like a dog. I plan on setting up a grouper tank again but I will not do so until I can set up a 300 gallon or larger tank and plan on leaving it with just the groupers for many, many, many years.
I'm sure you've read above, but just to reiterate, make sure your friend feeds a wide variety of foods and at a minimum skims the water heavily. I also ran a UV sterilizer to prevent parasites which I did have a problem with at one point.
Good luck with the little guy and I promise he will be one of your favorite fish from now on.