did he use sanitary procedures? ie. swab the vial with alki... use a different needle then the one you drew with... swab the inject spot... clean hands? etc etc... also maybe your buddy didnt have steady hands and was shakin the needle around.. this can cause the needle to tear some muscle around and become sore
as for treatments just take some antibiotics and keep an eye on it make sure it doesent form an abcess
another question........ whys your buddy pinnin you? lol