The kids are grieving the loss of their mother. Maybe they think their dad is responsible somehow for being on the road at the time of the accident, or for some other obscure reason. We all know grief messes with logic, and perhaps it is messing with the dad’s logic as well.
Perhaps the older kids don’t actually their blame dad for mom’s death. It might just be too ‘hard’ and painful to see him right now, particularly if they’re not used to seeing him broken and crying. Or perhaps they need to be ‘holding it together’ while dealing with their own work or studies.
If you’re a family friend, try to gently remind the adult kids that dad has to live with what happened. If he’d died, they’d be grieving his loss, but he didn’t. It seems unfair to make him feel guilty for surviving.
However, don’t lose sight of the fact the kids are in an emotional whirlpool right now. Everybody’s hurting; there’s no right and wrong. With time emotions won’t be so raw and hopefully, the dad and kids will be able to talk and understand each other better.