I pretty much agree with Kaya dog. The first time I dislocated, my friends tried everything they could to get it back in, and they totally destroyed my shoulder. I really wish that they had not touched me! After that first time, my shoulder was so damaged, that it popped out dozens of times a year, and I could always get it back in myself super quick, or it would just slip back by itself. It is unlikely your friend will be right-side up with a dislocation. Mine would pop out when upside down, trying to roll, right when I flipped, or doing a brace or boof stroke right above a class V rapid.
This is an instant swim. Trust me. You can not roll or paddle if your shoulder is out of socket. Swimming one-armed sucks, but what can you do? Your "consideration" in this situation is to help get him out of the water ASAP because he won't be swimming well on his own. Like I said, his shoulder will probably/hopefully pop back in on its own, but swimming will still be an issue even after it has popped back. Sublexations aren't as bad, and he should be able to paddle himself to shore after one.