Since it may take a while for the man's insurance company to contact you I would recommend that you commit what you remember to paper now. Dates, times, weather, road conditions, etc. It will mean that it is all fresh in your mind, also if it is a road that you regularly use, you have the opportunity of taking a look and including any relevent road markings, turnings, landmarks etc to include on your diagram. If you remember something slightly differently at a later date, it may not fully support the chap whose car was hit and possibly bring doubts as to how accurate your witness statement is.