If she isn’t a US citizen (children of diplomats generally are not considered to be so, even if born in the USA; the Obama and Trump admins both concluded this), then don’t let her back in the country.
And if she is a US citizen, then I would have federal marshals literally waiting at the airport gate to arrest her for treason the minute she sets foot back on American soil…to me, going over to ISIS is tantamount to exactly that.
Read on.