“Under the laws of Moses, they would have been stoned to death.”
Incorrect. Under the law of Moses, this girl and her boyfriend would be required to marry with no option for divorce. However, the absentee father of the girl would have had veto rights to the marriage.
Of the many infractions that warrant capital punishment under Moses, this is not one of them.
When people reject God’s laws, they will necessarily substitute laws which are either more lenient or stricter than God’s.
I like that solution... Marry her off.