If it was an at-will relationship, the employer can end the offer for pretty much any reason, as long as there is no violation on the basis of a protected status (age, gender, race, religion, etc.) Depending upon the circumstances, the employee may be able to claim detrimental reliance (e.g., abstaining from seeking alternative employment.)
(We're not in the position to judge the reasonableness of the employer's actions, as we don't know their side of the story and haven't seen the emails.)

