Wow! Sometimes….just when you think you’ve heard it all!
Think you may be more concerned about fairness to first or second families, rather than focussing on the most important unarguable fact. The fact that HE is being totally irresponsible. She is also being totally selfish and irresponsible. Her Mother the same.
You can’t help that, other than to insist you and your children aren’t to be put at risk as well.
He should stay where he is. His Daughter (isolating) remains where she is. Alone. Rather a lot of people are having non-ideal Christmases!
But, if he’s hell bent on showing he’s lost his mind, then he can of course drive to hers and stay with her until they are both free of Covid and it will be safe for him to return home.
If the ex-Wife is happy to have Covid then she (not some poor, innocent cab driver) , needs to go herself and stay isolated, until clear, with her own Daughter. Staying in a hotel? Travelling elsewhere? Given the facts, how many more innocent people does he plan to involve.
Stick to your guns, OP. You deserve better.