There is a box about "mundane spell effects" somewhere in the book. Simply put: a spellcasting character can do small spells without any effort. Making light, lighting the fireplace, generally, things that don't really have an impact on the narrative (Harry could flick a lightswitch or take some matches, but it is just cooler for a wizard to do it with magic).
If you are in a conflict however, that does not apply. So in your situation, the hidden character would have rolled a stealth "block", he is hidden. He probably tagged some aspects ("dark shadows") for example. Now the PC come in and you let them make an alertness roll to see if they can spot him. Since it is dark, the wizard might make some light, but since this is a conflict, he would have to do it as a maneuver. Maneuver spells in general last the entire scene, so you don't have to put extra shifts in duration. The new aspect ("light") now cancels out the shadow aspect, and since it is gone, it can no longer give a bonus to the stealth roll, which might be enough for the players to see the hidden character.
If you are casting against a darkness spell, the higher powered spell should prevail. A maneuver needs at least 3 shifts, but you can put in more to increase its power, so it is harder to resist it.