It would depend on the job as to whether or not that is possible. If he works retail, or a similar position, then his shift has likely been covered by someone else and they may also need the money being provided by that days pay and be unwilling to switch back out. If he wasn’t on the schedule for that day at all, rather than switched shifts with someone, than he would not be allowed to work unless they were severely short handed because retail positions are stingy about store hours and will rarely allow two people if one can handle it (metaphorically speaking). If he works in an office, he may be allowed but that depends on the manager and also what’s going on that day. If the day is booked full of meetings he may not be allowed to come late simply because he’d be interrupting the day at a midpoint. To be clear, I’m not saying they shouldn’t ask if they can go ahead and go in if it is a possibility. I’m just saying that in many jobs and many positions you can’t just decide to no longer be off on the day you requested off without some kind of work around, like someone else bailing on their shift leaving things short-handed.

If he wasn’t on the schedule that day then he would not have had to take the day off work.

I