If your student is switching for the sole purpose of improving the chance of getting into a more competitive college by going to a more competitive high school, then the chance will probably increase. The admission committee will view this move favorably as your student is taking the initiative to seek out a better education.
If the switch is for other reasons, then that's okay, too. A lot of students use a move as a way to help shape their story in their applications. A new high school can offer you the opportunity to try new subjects and extracurricular activities or learn about a different part of the country that you would not have been exposed to in your previous high school.