adjective

disloyal, especially to a spouse or partner.

her faithless lover

unfaithful,disloyal,inconstant,false,false-hearted,untrue,adulterous,traitorous,treacherous,perfidious,fickle,flighty,untrustworthy,unreliable,undependable,deceitful,deceiving,two-faced,Janus-faced,double-dealing,double-crossing,cheating,two-timing,back-stabbing,double-faced