This is definitely one of those awkward things where "the talk" needs to take place so there are no misunderstandings. For example, she may think there is a relationship of sorts, and he thinks they are just being friendly and hanging out and talking. (or vice versa, as the case may be.)
Sooooo, I don't really know who should be the one to bring it up.....but I don't think there is an official 'couples' status until it has been discussed, and there is intent on both sides to exclusively be with that one individual to see where things may go.
As I said earlier, a bit awkward to initiate the conversation....but pretty inperative so there are no misunderstandings and clarification.