Thanks all for your responses, they've been useful. A few answers to questions I think I was asked (and excuse typos / brevity - doing thumb-typing on the app while dreamfeeding)...
He's definitely her fiancé.
I searched for her (on the web, not specifically FB, but that was the top result and totally open) - this is standard practise though right? I feel no sheepishness about searching for her digital footprint - I would also always search for someone I was interviewing at work, let alone when that person is going to be in my home and in sole charge of my children.
I didn't search for him but he's tagged all over her profile and I clicked through, hence my rabbithole comment.
No, I searched for other candidate afterwards (who I also really really like and who DH always preferred) but couldn't find her - I get the point about what I can't see still might exist but as I can't see it I can't know.
I guess either DH or I were always going to have to slightly compromise and it looks like he 'wins' (although, like I say, there was just a hair's breath in it).
Oh and on a second look I can see she has liked and commented (usually with a 'haha' type statement) on a number of his rants so I feel on firmer ground assuming we're not culturally compatible.
I'm surprised though. I really liked her. But I really like the other one too so lucky me.