Hi J. - Goodness.... there is so much I could say here...!!
But I'll try to be short and sweet.
First and foremost, in every situation we go through in life -especially negative or hurtful - there is always a gift. Always. So look of the gift in this. (This is the gift for you!)
I saw it immediately. You child's dad is teaching her that there are all kinds of people in the world (i.e. ones that make people feel good, ones that make people feel bad... ones that make good decisions, and ones that make bad... etc.). In her life - as in yours - she will come across all these kinds of people.
In most cases, it is her choice as to who she wants to be around. SO - here's the perfect example of a bully. As she goes on - to elementary, middle, high school - she will come across people like her dad (this is not a specific word to use during this part of the conversation.)
If she is raised as a thoughtful, compassionate child with decision-making skills, she will look at a bully with a different eye.
Bullies are sad and lonely inside. (By this time, the conversation should be long away from her father - you get the drift.) They hurt other people because they are hurt inside. And some just don't know who else to be...
So that's the first thing that came to my mind.
Next, it's ok to tell her that when you fell in love with her dad, it was a bad choice on your part - for exactly those same reasons... You made a choice to not really see those parts of his personality that makes him a not-so-good-father. "He's not a bad person" (CRUCIAL WORDS)- just not right for you or the example you'd choose for her, but darn it - he is her father.
Here is a lesson in acceptance. We will have a lot easier time of it if we "accept, don't expect." Because All Stress in Life Comes From Resisting What IS... The man is who he is - by his choice.
This child needs to 1)accept the fact that he is who he is, and 2) accept the fact that she needs to go there and look for the good in him. Because this is a choice she did not, and cannot, make for herself.
And so - onto "different houses have different rules." Again, this will serve her her whole life and especially through her school years.
As she goes out in the "world" - playdates, school, etc., she will find that the rules in your house (about everything - including treatment of others, in this case. In other conversations, as it suits the situation) will be different than in other houses.
So this is where your training of her begins. In thought process, consequences (eeesh - you're not with her father anymore because of the choices he made in who to be in the world...), choice of friends (consequences thereof), etc.
Next, it's an opportunity to teach her respect. Respect that everyone has the option to choose who to be in the world. He's chosen a way, you've chosen a way, her Dad now (your husband) has chosen a way to be in the world. And then WE get to choose if we want to be friends with them or not.
(Yes, that relates to more than a forced visit with someone, but that is a situation created long ago and here's an opportunity to teach acceptance. Hey - she may be a gift to him in the long run!)
And lastly - like there's not enough here! These conversations happen BEFORE she goes. They're pre-emptive. This is about life, not just a weekend visit. But how wonderful that those visits can present you all with such gifts and opportunity for you to teach your child: Acceptance, tolerance, compassion, thought and decision making process, choices, respect - and our greatest commandment of all - LOVE.
WOW! Here's is the silver lining....