In order to get a gift via Mystery Gift from a friend, the gift
will need to be sharable and your friend will need to share it
however all sharable gifts will have a specific number of limits
the friend can share it. Also Nintedo of America never distributed
anything that was sharable, Nintendo of Japan is the only one that
allowed an event to be sharable and the only one that they allowed
was the Secret Key for their Pokémon Platinum players. In the event
of a friend sharing a gift, you would need to select "Get from a
friend" in the Mystery Gift menu." Mystery Gifts are not sharable
after Pokémon HeartGold and SoulSilver.