Your friend got lucky. The output diodes of the alternators 3 phase bridge probably conducted through the stator windings of the alternator and kept the voltage from going too high in the reverse polarity direction before the fuse blew to disconnect the incorrectly applied voltage, so that the electronics of the vehicle (computers, and the audio system) did not get destroyed.
If in the future if the battery goes dead when the vehicle sits for several days without being used, have an alternator rebuild shop check the output diodes for one of them failed electrically leaky in the reverse direction.
If the vehicle does not keep the battery charged enough one of the alternator output diodes may have failed open.
The bottom line is that the alternator took a beating if the battery was connected reversed even for a very very brief time. If the electrical system shows any hint of a problem in the next year or so, have an alternator rebuild shop go over the alternator.