Well she isn't the only trainer who has an evolved Pokemon when she shouldn't. Lance, Ghetsis and Iris are just a few others.
The reason that these trainers have these "illegal" Pokemon is simply to give you more of a challenge while still using a more intimidating Pokemon against you. It also makes the player feel more motivated to take the opponent down because they have a Pokemon that the trainer cant have. In this case, the creators also wanted her to stand out between similar galactic grunts also using glameows, so she wouldn't be too easy and ordinary.
Hope it helped!