If he has no pit sensors, the driver could be triggering the pit menu manually... accidentally.
Check the "Secs before open pit menu" on the main page, and make sure it's at least 1s, if not 2s.
Also, check the "Button for pit menu" option. If it's set to just brake, and your pit menu access was less than 1s, then he could easily have been entering the pit menu when using his brakes. Similarly, if his controller was dirty or bad (set controllers can malfunction this way), then he might be getting the problem without even pressing the button.
If/since you're not using pit menu stuff, I'd jack up that pit menu delay to 10s or more, so that it literally never happens in a race. Then, the button option won't even matter.