A fully canceled trip will be a flight credit that cannot be used by anyone but the original ticketed passenger. A trip credit is what you get when you book a new ticket with a flight credit, and there is money left over. That residual is given as a trip credit that can be used by anyone.
Flights originating in Europe, however, are not eligible. This may be a ticket she loses all together unless she purchased trip insurance that would cover her medical issue.
https://www.aa.com/i18n/customer-ser...vel-credit.jsp
**When booked on aa.com, Flight Credit can only be applied to flights within the U.S., Puerto Rico, and the U.S. Virgin Islands. (U.S. currency only)
OP should be able to use a flight credit for another flight ex-LHR but would need to call -- can't be done on aa.com. Same passenger rule still applies, though, so the ticket might be lost nonetheless.