Hoo boy.

[Ocelot II]

Not much you can do to talk him out of it - the harder you try the more he'll dig in -- but make it clear that there's a two-way street and if she's imposing conditions you will impose some of your own, e.g., we'll babysit only if xxx; if we can't see the grandkids we won't be providing any financial support for them (college funds, inheritance, etc.); we won't bail you out financially if she gets into money trouble. Write it down. If he goes ahead anyhow and she takes his money and his kids, it's on him. For sure he should try to get a prenup, though I doubt she'd sign.