With a gift this may be tricky, but such unusual damage occurring on a new piece... I would bring it back to the seller and expect repair or replacement. It may now work, but this could also be way easier than you might think - depending on what the seller makes of it. Most would service manufacturing defects - and I imagine that there must have been some for the pearl to leave the post so easily. Usually it is a pain in the b^%% to take pearls off posts even when you need to
2c
Wouldn't try to repair the pearl. Even with a solid pearl (i.e. all nacre - natural, keshi or tissue nucleated etc.) would be one dicey and traumatic exercise unless those are just the tiniest scratches.