This is not a prank call. The caller is a criminal intent on defrauding your MIL. The FTC warns about the family emergency scam here http://www.ftc.gov/bcp/edu/pubs/consumer/alerts/alt111.pdf
Report to the Internet Crime Complaint Center http://www.ic3.gov/complaint/default.aspx , the Federal Communications Commission http://esupport.fcc.gov/complaints.htm , the Federal Trade Commission https://www.ftccomplaintassistant.gov/ and your state AG, as appropriate.