Originally Posted by mcgahat
If you are on seperate tickets, you are taking a chance and the risk is one that you take.
Very true!
Unfortunately travel agents and even tour operators issue seperate tickets for connections without warning the passenger.
But in this case, according to OP it was a group booking and the UA ticket showed the endorsement "INT CONJ TKT NO….", which means that UA was aware of the conjunction between the two tickets.
I don't know about US legislation regarding domestic flights, but if it would have been an international flight under the Montreal Convention, UA would have to pay any costs occured because of the delay up to ~5000 $ unless they could prove extraordinary circumstances beyond their control.