Given those qualifying as friends would be determined from your social media accounts (=a lot of friends), I think "facing each other" situation might happen too often and cause your phone beeping, vibrating, and annoying you many times per day. You turn all 360 degrees once every hour at least, if not more often. Because of the latter, even if you chose just one person as a "target" for directional facing, it still might happen too often, no matter where they are in the world.
This probably depends on the precision of angle of the straight line between two people that has to be met to consider directional facing taking place. If two people are far away (different countries or even continents), the angle precision to be met can go to great lengths, since a person then is super small compared to the distance between him/her and their friend abroad. Any minuscule deviation from the straight line would then cause two people (whom in this case becomes two super tiny dots) to directionally bypass each other.
Therefore I suggest imposing more criteria that have to be met in order to get notified about your friend directionally facing you. One thing that comes to mind is two people keeping a certain angle for facing each other for a long enough time, e.g. if you both walk or otherwise travel in the direction towards each other for some time, say a few minutes.