What is the ruling on this married man's dealings with his female colleagues in the mixed-gender workplace, and his frequent and detailed mention of some of them to his wife?

The mixing of men and women in workplaces is a great gateway to fitna (temptation/strife) and a cause of corruption. The original principle is to avoid it. If a man is compelled to work in a mixed environment, he must avoid the causes of fitna and uphold the limits of Islamic law in his dealings with women. His speech with them should be only when necessary and to the extent of that necessity, and he should not elaborate unnecessarily, because speaking with a young non-mahram (definition) woman without need is a cause for fitna. The scenario mentioned in the question indicates excessive and lenient speech without a valid need. Therefore, the wife should advise her husband gently and wisely, and warn him against leniency in dealing with non-mahram women.