I can't see any reason for Greg's girlfriend to have any contact with his ex-wife. Greg should be handling all calls made to her about the kids. Greg should be picking them up. He should be disciplining them. His ex- can't "let him go" as you want because they have kids together and will always have contact unless you want those kids to be miserable. You shouldn't be anywhere near her so her wanting to fight won't be an issue. If Greg can't pick up the kids alone or without you, he should make other arrangements to get them. What you do is to provide a healthy, calm home when they visit and show them you like them by being nice. In other words, stay out of her face.