Well, now that the accusation has happened, you can either have your own sim apologize (for calling his fiancée out on her behavior? Yes, that seems backwards) and try to build the relationship back up, or you can cut your/his losses and find someone else.
Or, you could move her into your active household, cheat up her lifetime happiness points, buy her the Clean Slate LTR, and wait until your sim calms down enough that they can talk it out. (You might have her apologize instead. What a thought.)
Or, I guess, you could revert to a prior save. Hopefully you have several. If not, though, the game keeps the file from the second-to-last time you saved, as a backup—it will be in Saves in your game folder in Documents with a .backup extension. Delete the extension and rename the save (so that it doesn't have the same name as the more current version), and it will be available to load at the Main Menu.
In the future, an NRaas DebugEnabler object reset on a sim will clear the action queue without resetting anything else. There are no accusations in my game unless I want them, especially given how unreasonable the standards are. If your sim's fiancée ever had a romantic relationship with someone else, even if it went no farther than a first kiss (or I think even a "confess attraction"), and even if it happened before your sim ever met her, that's still somehow grounds for an accusation of cheating all that time later, with no warning whatsoever.
Or maybe she was messing around behind your sim's back; who knows? But you could pull her into your active household and have a look at her relationship panel; if you see another recent romance, you can quit without saving and tell your own sim to move on. Just make sure he breaks things off properly before diving back into the dating pool, or he could be the one getting accused the next time around.