Huma Abedin's emails belong to her; the search warrant should be served upon her. Once that happens, she can challenge the warrant's legality.
The FBI has a history of abusing search warrants to illegally read Americans' emails. Did the agency just do it again, in the highest of all high-profile situations?
We don't expect our own government to hack our email – but it's happening, in secret, and if current court cases go badly, we may never know how often.
If a computer search would qualify for a warrant if its whereabouts were known, why should simply hiding its location make it legally unsearchable?