Is pushing a friend during his ablution to make way for a higher-ranking colleague, as a joke, considered an act of apostasy or a sin?

Your action is neither apostasy nor a sin. There is no harm in asking your colleague to make space for another to bathe, even if he has not finished his ablution. Making space is permissible and does not invalidate ablution, and there is no sin upon you for asking him to do so. Your pushing him away from the basin is also permissible if you know that this action does not harm him due to your usual joking. We warn you against obsessive thoughts (waswas).