I have a close friend who I have been friends with for years. We meet up often, usually on a weekly basis, as we had DC at similar times and similar age gaps and they enjoy playing together (hers are 5 and 2, mine are 5 and 3)
She split up with their dad when her youngest was a few weeks old. In the two years since then, she has had 5 boyfriends. Within a week of getting together, they have moved into her house, or she has uprooted the kids and moved in with them, has gushed about how they are 'just like a dad', calls them a family, posts things on facebook. about how biology doesn't matter, it's in the heart blah blah blah.
Then without fail after 6 months maximum, but usually after a number of weeks - the relationship ends up abruptly and the kids are disrupted again.
I've tried raising it in the past but she shuts me down with how happy and adjusted her children are.
We met up last week and she was telling about her new man. I sound like a terrible friend but all I thought was ok here we go again.
Now she is all over social media and texting me calling him her soulmate, how he's amazing with her kids. He met her kids two days ago and she is posting pics of him with them and what a fantastic dad he is being and how they have a lovely bond all ready. 
I feel so sorry for her children. I feel like they must be so confused. Do I try and talk to her, or just accept this is how she is and distance myself? she has been a close friend for years and I have known her children all their lives, and see them often so as strange as it sounds I feel a degree of responsibility for them and I care a great deal about them.