She should have taken Southwest Airlines, which has a wonderfully specific policy on the issue.

[link to www.southwest.com (secure)]
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Customers who encroach upon any part of the neighboring seat(s) may proactively purchase the needed number of seats prior to travel in order to ensure the additional seat(s) is available. The armrest is considered to be the definitive boundary between seats; width between the armrests measures 17 inches. The purchase of additional seats serves as a notification to Southwest of a special seating need, and allows us to adequately plan for the number of seats that will be occupied on the aircraft. In turn, this helps to ensure we can accommodate all Customers on the flight/aircraft for which they purchased a ticket and avoid asking Customers to relinquish their seats for an unplanned accommodation. Most importantly, it ensures that all Customers onboard have access to safe and comfortable seating. You may contact us for a refund of the cost of additional seating after travel.
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I myself strongly disagree with the policy of giving away an extra seat, but I understand that the airline is trying to avoid politically correct lawsuits. The crucial point of Southwest's policy is that every passenger has a right to armrests as physical boundaries of his personal space.