How "ok" is it to allow NPC's to cast spells that aren't in the rulebook?

I'll give you an example.

One of the villains of my story is currently being transported via boat through a river while being bound with chained metal gloves and a mouth covering guard (so he can't cast any spell), to let him eat they remove the mouth piece temporarily and let's pretend he has found a way to free himself from the gloves but conceal it from the guards.

So when everyone is asleep he finds a way to appear in front of the adventurers miles away to convey a message. He's not physically there.

I'll be honest I don't remember all the spells that are in DND but I'm pretty sure project image would work, not with that range and without line of sight.

I guess it makes sense on a narrative level for a powerful wizard to do things that others can't but one has to ask, if I don't follow the rulebooks what makes a player just say "yeah but why can't I do that too"