Ford watched Denholm with sharp, wary eyes. He didn't think the man had any reason to betray them just now. He couldn't escape from the castle; wights were everywhere.
He also couldn't kill any of them and expect to walk away with no consequences. He was a coward and a snake, but for the moment, the worst he could do was deliver harsh remarks and biting sarcasm.
Unfortunately, Denholm was very well practiced with using words like knives.
He knew just where to aim to get through Ford's determination to remain calm. And yet, Ford remembered Denholm's temper from the cave, long before.
He was not immune to the power of words, himself.
Ford chose not to provoke him at the moment. So long as Denholm was silent and doing something at least of nominal use, Ford decided to leave it be.
Mia had apparently decided the same.