Then how is she contacting you? If shea texting or calling then tell her to cut it out. Tell her you aren't interested in a relationship with her and to not contact you again. You wont have to see her or hear from her then issue is gone. What happens between your partner and her is not your business and tell him to keep their relationship to himself.
Sue, openly in a vee with Nate (polysexual, many fwb) and Sam (Mono)