"...he has so little self belief/self worth that she honestly thinks that by him "choosing" her it must mean she is worth something..."
^^This.
Many years ago, one of my friends was dumped on by a bloke whom we went to school with. She was so enamoured with this bloke that she even followed him into the same university - then couldn't understand why he wanted to date/sleep with other women whom he met there, rather than with her. She started dating one of his new friends, purely in an effort to make this bloke we went to school with jealous. In hindsight, I think he must have known how my friend felt, but simply continued - in a self-absorbed sort of 18/19 year old way - to ignore it. My friend's boyfriend fell very deeply in love with her, proposed, a giant wedding was planned. We must have been 20/21 at this stage. The boyfriend was, actually, quite lovely - he clearly adored my friend, he made an effort to fit in with the rest of us whenever we met up, he was good with small children, her family all loved him...
Within 6 months of their expensive giant wedding, my friend was cheating on her new husband - with a bloke who worked in her office. This man wasn't married (I don't think), and there were no children involved (thank God), but my friend? Totally obsessed with this new man. She'd finally worked out that the bloke she'd followed to university wasn't interested in her (actually, her "love" turned very quickly to hatred once she'd grasped the fact that he wasn't going to stand up and declare his undying adoration for her, in the church where she was marrying one of his new friends - because yes; he was a guest, he took his now-wife as his plus-one, and my friend kept eyeballing him hopefully), and she wasn't interested in her husband.
I don't actually know how her first marriage ended, because I said that I wasn't getting involved. I do know that within a few weeks of her divorce coming through, she and the man she'd been cheating with were married, in a very quiet/no guests other than her parents civil ceremony. And having seen the photographs? Her parents looked absolutely stricken. My friend gave up her career, went through IVF round after round in order to give her new husband the child he wanted (although I know how deeply she loves her son, I do wonder if he's only here in an effort she made to cling onto the man from her office)... and very quickly learned that even becoming a father didn't mean that her new husband was going to give up the single life.
They're still together, 10 years later. But my friend isn't happy. She suspects that her husband is having affairs, but... rather brutally perhaps, she doesn't get the sympathy she wants from any of us, because she did exactly the same thing that her husband's currently doing. And almost 25 years after leaving school? She will still turn the conversation around to the bloke she followed from sixth-form into university.
Her first husband, meanwhile, is blissfully remarried with a young family of his own. I bump into him every now and then, and although we pretend not to recognise one another, I cannot help but be glad that he's with a woman who clearly loves him as much as he does her. If he'd stayed with my friend? I think he might always have been second-best to the bloke she was obsessed with, who is still friends with him (and still seemingly oblivious to my friend's obsession with him). If he'd dated/had sex with my friend at university, then perhaps so many lives wouldn't have been tainted by her lack of self-worth. She'd have gotten over her obsession, moved on, been happy and able to return someone else's genuine love for her. Her current husband, I don't think, ever wanted to get married. I think (he's a lot older than we are) he was perfectly happy/suited to playing the field. Their child wouldn't be caught in the middle of sneering, disdainful parents.
Why doesn't my friend leave her second husband? I have, actually, asked her this once, when she was crying on my shoulder about his latest flirtation with someone "younger, prettier, thinner" than her. Because she knows that she almost broke her parents hearts when she abandoned her first marriage, and she doesn't want "to disappoint them again". It's all very sad, but it's of her own making. And yes; our circle is always going to catch her when/if she jumps... but not one of us condones her behaviour, even though we don't condemn her for it. She was young, very immature, and made a lot of mistakes that we couldn't talk her out of making.
Your friend sounds very alike to mine. All you can do is refuse to engage with her boasting, try to boost her self-esteem as much as you can, and catch her when this blows up in her face. Because, one way or the other, it will. She'll either end up trapped with resentful/angry stepchildren in her life, or alone, knowing that he always thought his wife and family were his priority, and not her. She's going to end up hurt, either way.