Some similarities with what happened to a friend of mine..
Met a lad in a pub, he's about 5 years older than us... they get it together, he mentions he's living with his mum having broken up with his ex, who has a kid who is not his but he's known almost all of kids life...
They go out together, she (and i and our other friends) meet the guys mum, his dad (his parents are separated), all lovely, he sometimes goes away on holiday with his best mate who we also know, sometimes he stays at his dads flat who he works with...
This all trundles on for 3 years... as far as we all know they are a couple, we know everyone in his circle, friends and family.. all is well.
He gets his own flat, she to all intents and purposes moves in with him, around 6 months later, I moved into the flat upstairs from them.
Again, all is well, hes home a lot, he works with his dad, occasional weekends away with his mate or he gets drunk with his dad and stays in his dads flat (other end of the block from ours!)...
One evening I bumped into him in a carpark down the road from our flats - he's sat on the bonnet of his car talking to a woman I don't know. I wave, he waves, I go on my way home...
Then one day I hear knocking on their door, I came down and theres a woman I don't know, but she looks like the woman from the carpark, knocking on... I say 'oh, Im sorry I think he's at work and she's up at her mums today, can I give them a message'...
And she's reallllllly cagey... 'I want to talk to x... she needs to know something'... So I text her and said er... i think you need to come home, and woman goes to wait in her car outside.
Yeah. She's the other woman, or rather, they both are.
He's been telling her he's living with his mum as she needs care, working with his dad, has barely any time but he spends several evenings a week at hers and she thinks shes in a committed relationship with him, been on holiday with him etc etc.