You can purchase the policy before she arrives.  When you
purchase the policy, you specify the dates the policy begins
and ends.

Some companies that I've come across:
http://www.specialtyrisk.com/
http://www.virginiarisk.com/visitorsinsurance.htm
http://www.travelinsure.com/who/visitors.htm
http://www.isis-travelinsurance.com/
http://www.psiservice.com/
http://www.imglobal.com/