IMHO a person's ex is a matter for that person, not any subsequent partners, boy/girlfriends or sundry family members, hangers on etc. All communication with an ex should be by the ex's ex-partner/spouse unless it concerns specifics like looking after any children during access periods, planning weddings of children of the first relationship (when the new partner may have some input but by invitation only).
Unless the circumstances are exceptional any new partner must keep away from any and all communication. Taking sides in a dispute will only serve to make matters worse and in the end the new partner will find themselves marginalised as the two exes become resentful of the newcomer involving themselves in matters that they were never privy to when they first arose.
Oh and as for why bother asking? Advice is meant to be given not taken.