Morning gman,
What do you mean she's just been dragged into the office? At that time of night?
I'm not in HR, but I do know that HR would have to request to talk to her and she would be given the opportunity to take someone with her to witness what is being said (though they are not allowed to put in their tuppence worth). After your wife has given her side of the story, it's down to HR to decide if there really are grounds for an official warning.
If she receives a verbal warning, it will stay on her HR file for 6 months, if it's a written warning, it will stay on her file for 1 year.
Your wife is perfectly entitled to dispute the warning.
If her employer does not have suitable evidence or any grounds for thinking it's true, then they will help to iron out the issues between your wife and her colleague.