Does the entire family travel on a special passport, or only the diplomat?
If her husband was in the foreign service or one of the numerous agencies, e.g. US AID, attached military, etc.
She will not face British justice... But she may be subject to US Law and our legal system.
Many nations have laws that subject foreigners to primitive punishment by Western or USstandards.
Dip Immunity affords protections against prosecution in that country but the US does process these cases.