I don't see why you shouldn't be able to book on AA codeshare. However, the codeshares do not appear on availability for the market (because they have no authority--just for connecting/stopover traffic) so that's why she was unable to find it. You need to ask to do a "long sell" and provide them with the AA flight number. You can get the latter from the downloadable timetable at www.aatimetable.com but only on the City/Airport Timetables screen (the blue airplane logo) (again, same issue here).