This is a very risky plan
Your colleague has already beaten the odds by getting a tenure track position, and there is no guarantee that they will be so fortunate if they decide to start over. We all like to think that our success is due solely to our own talent and hard work, but chance plays more of a role than we usually want to admit. Leaving a tenure track position for a postdoc means taking another spin on the wheel of fortune and accepting the new result, even if it's not as good as what you've already got.
Beyond that, I would expect your colleague not to be a top candidate for the postdocs. Presumably there are students graduating every year who have done their PhDs in field B, so why would someone hire your colleague with no relevant experience instead of one of them?
Furthermore, your colleague is now overqualified for a postdoc position. Often people are reluctant to hire overqualified candidates because they are concerned that they will quickly become unhappy in the junior position because of its lower pay, lack of autonomy, and lesser prestige. Maybe your colleague would be an exception, but the prospective supervisor has no way of knowing for sure. Hiring someone with appropriate qualifications will be seen as a safer choice.
Finally, after all of that, even if your colleague manages to get a position in field B, and even if it goes well, then what? We all know what the academic job market is like. Would you really advise someone to face that again?
There is nothing stopping your colleague from pursuing work in field B, especially once they get tenure. It will involve some reaching out and making connections, as well as some self-study, but they don't have to do it all at once. They can still be productive in field A while training up in field B. People do it all the time; you don't have to go back to apprenticeship every time you want to do something new.