A similar thing happened with my grandad (dad's dad). We were very close, my dad's mum died when my big sister was just a few weeks old. My parents lived a few doors away and for months my grandad was barely coping with the loss of his wife so my mum cooked tea every night/housework etc.
He always came on holiday with us and we saw him a few times a week.
When I was about 6 we suddenly saw him less and my parents found out he'd met someone.
My mum was really happy for him, didn't want him to be lonely but really my dad wasn't great. He felt like his dad was being unfaithful to his mum in some way, even though she'd died 10 plus years ago.
For years and years we hardly saw my grandad, he completely absorbed himself in his partners family, she had a big family and grandkids etc.
We felt quite bitter at the time but in hindsight I can see that yes he did leave us, but also my dad wasn't exactly welcoming to his partner so didn't make it easy to do things (he wasn't mean, just standoffish - quite ridiculous either way).
Anyway roll on 30 years and they were still together, he lived in her house, really happy together. She went in for a routine op which unfortunately went wrong and she went to ICU and died 2 weeks later. Again my grandad just shut down, he was devastated and almost silent.
Her family seemed to take this that he didn't care, and asked him to leave her house as they were selling. This was within a couple of days of her dying.
He went to live with my parents and about 10 days after she died he became very ill, massive PR bleed and was taken into hospital. He went on to have a cardiac arrest and died 2 weeks to the day after she did.
The cause of death was officially DVT but unofficially we know he died of a broken heart. He couldn't cope with losing another love.
Her family still weren't great. They asked if they could have his car, they'd bought it 50/50 I think but they wanted it for one of her grandsons. The alternative would have been to sell it and split the money but it wasn't a particularly fancy car and my parents were to full of grief to bother with messing around with it so just let it go.
They were allowed to the house once, for 1 hour to take some of his things, but it was very much my grandad's things only. They came away with a few of his tools from the shed and a lovely rose bush (thankfully in a pot). We put his ashes in the pot and the rose thrives every year which is lovely to see.
So, if you've kept up with all of that!, it was shocking to see, like you're seeing, that after 30 years of togetherness he would have been left with nothing and the family that we feel he left us for, actually just kicked him out. He'd been at all their family events, Christmas days, birthdays, christenings, holidays, he saw them almost weekly. It meant nothing to them in the end.
I'm sorry for what your dad (and you) are going through.