He should approach HR, if it's not his work performance then they could supply reference but if it was his work they may well side with his boss, alawys two side to a story and people only tell us their side, not saying that your cousin is in the wrong, but as a manager I can say I on a small few occasions have had poor work performance by employees blamed on the interaction of the manager, some people can be difficult. But if this not the case for your cousin then defo he should contact the HR dept directly.