Lets say you have Slowbro against a Gourgeist
The Gourgeist used Trick or treat which will get a ghost type to your slowbro
And then your slowbro used Curse
Will curse cut down the slowbro HP and the gourgeist is cursed?
In the example you gave, Curse will cutdown Slowbro's HP and Goureist will be cursed.
I used LC mons but the effect is the same.