PLEASE tell your friend what she NEEDS to hear even if she doesn't WANT to hear it.
As a sahm she is extremely vulnerable financially and legally (which it sounds like you are aware of)
Anyone familiar with my posts on the subject will know I have witnessed a relative royally screwed over as a result of being an unmarried sahm living in a family home that was solely in her partners name. Not screwed over by the partner though - but when he died his family screwed her and their kids over, pretty much unceremoniously turfing them out of their home and putting her in a position where she had to return to full time work while they were all still grieving.
It's NOT just in the event of separation that things can easily go awry, if your partner becomes incapacitated or dies, especially if they're the breadwinner and even more so sole earner you can be left in a seriously precarious financial and legal position.
Loads of things are just so much easier to administrate if you're relationship is legally recognised
"but obviously has legal implications which are of benefit to the lower earner/party with fewer assets." There are benefits for both parties eg the higher earner usually the man gets parental rights recognition with marriage.
"It is blatantly unfair that this happens." Totally agree on this that the state doesn't have a clear setup on this.
"I find that in most cases, when a man says he doesn't believe in marriage; it's not true. It's just that he doesn't want to marry that particular woman." That's very much what I've witnessed too. My ex claimed I was dragging out the divorce to avoid marrying OW, soon as I realised I made sure she knew it wasn't ME delaying anything!
I've witnessed people co-habiting for decades with the guy refusing to marry and giving it "it's just a piece of paper" bollocks, sometimes also delaying ttc (which is a REALLY shitty thing to do), then leaving their "partner" for another woman who they quickly marry and also often having children too.
They've treated their previous partner as nothing more than a placeholder until someone they considered more worthy of their commitment showed up!
We see it on here all the time, I've been flamed for saying so and so have others who've said similar - if he's not interested in making a genuine commitment within the first 3 max 5 years then he doesn't really consider you his life partner and worthy of his full commitment.
I was clear with ex from the start that I wouldn't hang around for someone not interested in making a full commitment and that I wouldn't be ttc before marriage, not engagement (which is meaningless, though I've seen it used to fob women off), but marriage. When we started dating we were both quite young still and it was general discussions of the topic not specific to us as neither of us actually thought we'd get serious with each other initially.
He proposed around 2.5 years in quite spontaneously after we went through a tough situation together, he didn't even have a ring prepared or anything. I half thought he was joking because of how it came about. We married about a year later.
When I look at my friends and family and their relationships there are very very few where my theory doesn't hold up - so far.
"then for your own sake make damn sure your partner makes a Will and you are the beneficiary." If you're not married they can cut you out of that will whenever they wish without telling you - I've witnessed that coming to light after a persons passed too.
I even know of one case where the "partner" discovered "their" man was married to someone else - and yes I mean fully married still in the relationship until he died at which point it all blew up!