I was in that position when I was TTC and my friend (well I thought she was) blurted out to me that she was pregnant in front of a whole bunch of people. To this day I am hurt that she hadn't done it more gently and considerately, especially when I had been confiding in her about our issues conceiving.

I would think the most considerate way is on the phone or preferably face to face, but somewhere just the two of you, ie not in a café, restaurant etc. I think you want to give her the ability to be happy for you and tell her face to face (as a best friend would ordinarily get to do) but also have it private enough that if she is upset she isn't exposed.

She will be very happy for you, but it will naturally be hard for her to hear. Good on you for being so considerate