Is this a case for slander/defamation?

My partner is a DJ at a club. A complaint came in about him from a man, saying that my partner has been buying drinks for, and flirting with, his wife. This is absolutely false, he takes $20 with him and there is no chance he would spend that on anyone else.(as for flirting, the woman in question is absolutely not his type, unless he goes for toothless, 380lb women) Now this man has been sending letters, complaining about my partner. He got spoken to by management. All he does is go and do his job, he lives for it. Now this man has destroyed his enjoyment, my partner is so wound up and astonished that someone could make up such horrible things with the intention of my partner losing his job. Does he have a case for slander/defamation? Especially if he loses his job over this?