If he tells Emma, he *is* asking for her to choose. And odds are she's going to choose her boyfriend.
She's not likely to say "Oh, that's so sweet" and go back to a happy platonic friendship. And if she's been with her BF for five years and seems content with him, she's also not going to confess her undying love in return and ditch the boyfriend to be with him instead. So the most likely case is that she sees it James making a move on her, is not impressed, and chooses her boyfriend instead of the friendship.
James has two things he can do. He can sit firmly on the feelings, so he can push them away and engage in the friendship in a genuinely platonic fashion. Or he can back off on one-on-one socializing with Emma - interact with her in a group more than just the two of them.
The one case where he could tell her about it is to explain why he's backed off - "I've realized my feelings for you are getting more than friend-like, and I need some space to deal with it." This can actually be a courteous option, if the alternative is hurting the feelings of a close friend who doesn't understand why he's suddenly dropped off the face of the earth. But it has to go with a clear message of "This is my problem, and I'm working on getting over it, and I don't expect you to reciprocate", which means he can't tell her and expect everything else to go on as usual.
How is James' social life otherwise? Does he regularly date, or get out to meet single women? Because I've seen it happen before where a shy person (male or female) who isn't active on the dating scene, or feels uncomfortable in it, tends to develop feelings for friends because they see them as a safe option - after all, this person already knows them and likes them.