If I understand things correctly...I think that what you're trying to say is…if you do a single click or a single finger tap with an input device (trackpad, mouse, etc.) on one of the drop down menu items…the drop-down selection doesn't stay visible unless you continue to hold down the mouse button or trackpad click or single tap.
Will guess that he has an app in Full Screen which hides the Menu Bar.
There is no such thing as a lock for the Menu bar - the Menu bar is already permanently affixed to the top of the screen unless you elect to view an app or window in Full Screen mode - which would then hide the Menu Bar at the top of the screen and will stay there unless you move your cursor up there or exit Full Screen mode.
Slowly move your mouse cursor toward the top of the screen and when the top menu bar appears, click on the opposing arrows. That will take you out of full screen mode and your menu bar will stay fixed at the top.