Best friends is definitely not the term for this. Best friends would have platonic love, or love between friends without the aspect of physical attraction coming into play. We already knew she liked him, and we see that he is at least noticing that he now thinks of her as something more than a friend. There isn't really a good term that I'm aware of for where they are right now, most of which are all or nothing compared to their situation, or include a physical relationship they simply don't have with each other. The best I can do is call them indecisive until one of them either makes the confession or simply accepts that this isn't going any further than friends.