With Mercury in Leo, conversation can become difficult when they feel they need to impose their point of view. Admitting a mistake is hard for them, and an attempt to correct them can make them raise their voice or respond with defensive gestures.
Correcting them in front of other people can intensify that reaction, especially in a setting where they want to keep an image of confidence. It is usually more helpful to talk in private, with respect, and to give them time to consider the arguments.
One way to put it would be: “Look at this argument calmly and take your time. I don’t want to force you to think a certain way.” Even after reconsidering their position, saying so out loud or apologizing may be hard for them and call for work of their own.
Intellectually, they like to be prepared. They tend to study their field of work and to make sure no challenge finds them without an answer.