Janet,
In principle, I agree with Dave's advice, but in this case it sounds like the two women in question must be fully aware of the situation, and are going along with it.  I think the letter writer should express her objections to her ex's behaviour, proceed to distance herself from him (assuming they don't share custody of dependent children), and leave her ex and his sequential partners to their own devices.  Why those women would put up with such disrespectful treatment, I simply can't understand.