Don't feel guilty! Of course it's natural to- your best friend obviously means a lot to you, but crushes are feelings that you can't control, so don't beat yourself up about it, is my main advice. I think most of us have been there.
You may feel as though you are betraying your friend in some way, but as long as you don't act on it in, you're not.
The situation becomes complicated if you think she may feel the same. If this is the case, I would probably advise to keep your distance for now. It may be that their relationship runs it's course without any intervention from you.
If, on the other hand, you are sure that she is in to him, that's where it gets hard for you, I guess. Your friend may want you all to hang out together and it will probably be difficult for you to see them together. Unfortunately, there's not an awful lot you can do about this. But the great thing about crushes- they always pass!
I probably wouldn't advise telling her, although if you did, this is pretty cool way to go about it (and appropriate for Christmas-time!)