Add help text to radio button when it's disabled
I would suggest you might want to think about using the Conditional Fields module and having two separate fields for the change in Group B.

But for this specific question-- not knowing your HTML structure exactly -- you'd likely just be adjusting the HTML of the label. And that would then be more of a JS design question than a drupal one (unless you are building a modue around the idea) ... how to do that best depends on how re-usable you'd want it. In straight up JS: I'd likely store a hashed array of the label text, then have arrays of the option b input radio button ids -- one for option a1, the other for option a2. And then have a function that works on the appropriate array, passing in whether it should be enabled for not: foreach optionba1; change state, unselect anything checked, change text (return to original or add new text).