You describe a maneuvering board problem. You use the maneuvering board sheet according to the directions printed on it.
It is a quick graphic method for calculating relative motion between two moving objects. (Your boat and your friend's.)
It is an excellent way of calculating such things as the course and speed of another vessel or closest point of approach to it. Once the technique is learned, it can be used mentally for a rough but quick result when needed. Good stuff to know.
Here are links to places that can supply a pad of them and a manual for using same: