do_cool_things.py

#!/usr/bin/env python
from subprocess import Popen, PIPE
import time

import RPiGPIO as GPIO

MY_VIDEO_FILE_PATH = '/home/video_name.mp4'

my_process = Popen(['omxplayer', MY_VIDEO_FILE_PATH], stdin=PIPE, close_fds=True, bufsize=0)

GPIO.setmode(GPIO.BCM)
GPIO.setup(22,GPIO.IN,pull_up_down=GPIO.PUD_UP)
while True:
    button_state = GPIO.input(22)
    if button_state == False:
        print('Quitting the player now')
        my_process.stdin.write('q')
    time.sleep(0.9)

See omxplayer documentation