My friend placed Flareon and Ditto on the Day care.
The Flareon had Fire Blast, The Ditto had Transform. They got an egg. When the egg hatched the Eevee had Fire Blast. According to Showdown / This site / Many more sites. Eevee can't learn Fire Blast.
And I'm 100% sure that my friend didn't hack.
This was Gen IV PKMN SoulSilver. Glitch?