I've been on both sides of this fence ... had my daughter taken to my ex's new girlfriend's place and I was fuming. Truth be known I think I was jealous and wanted him back so made a big deal of it. When the dust had settled and I had moved on I didn't really mind because I knew he would be looking after her properly (I did lose it a bit when I realised the new girlfriend was being called Mummy too!)
The flip side of that is my new husband's ex was the same with me. I wasn't even allowed in the village because it was her village! I didn't actually meet his children until about 15 months later because of it. She changed her tune when SHE met someone and wanted him to meet her kids!!!
In essence, until people move on, in my opinion, they find a new lady a threat ... doesn't mean she has to find a man but it means she has to put her issues behind her and behave reasonably.