PLEASE FUCKING READ THIS.

I think i may have figured out the message the trainer gives you ("I fell to evil spirits despite my training"). It's actually something a Channeler says. A Channeler is a trainer that, and i quoute:

"have only appeared in the Generation I games and their remakes. They are generally depicted as Shinto priestesses. They scared out people from Pokémon Tower, and were possibly unwittingly helping Team Rocket make their base in the tower. Their strange behavior when challenging the player, and subsequent reaction after being defeated, seems to imply they were possessed by the ghosts they used while battling. In fact, some Channelers apologize after battle, saying "I was possessed!" when spoken to.
They specialize in Ghost-type Pokémon."

And actually, they say "I fell to evil spirits despite my training."
Here's an extract from liveblog: chapter 18 of pokemon yellow:

"{Moving on,}
Channeler?: Zombies!
{The Channeler unleashes a level 24 Gastly. Vulpix is lucky enough to win and grow to level 27.}
Channeler: Ha? I regained my senses!
Yellow: (Her possession must have been more subtle.)
{Yellow finds a Nugget.}
Channeler?: Urgah... Urff....
{This time, Pikachu takes on a level 22 Haunter.}
Channeler: Whoo! I fell to evil spirits despite my training!
Channeler/Haunter: Give... me... your... soul...
Yellow: yipe!
Pikachu: Never! You'll never take him!
{This Haunter is level 23. Pikachu blows it away with two critical hit Thunderbolts and grows to level 28.}
Channeler: Gasp! I was under possession!"