I'd severely limit it:
1) to the length of the wire. On a miss roll a d% vs 40% chance the bolt going over their head / to the side and extending past the copper wire, snapping it at the midpoint. Meaning the range will be halfed next time you try using that wire.
2)To hits that do a X or higher damage. Bolts have wooden shafts. If you deal X or less damage, the bolt won't bury itself in the target deep enough for the wire to touch the intended target. Maybe instead of damage numbers, treat it as concealment, so the player won't know if it hit hard enough when casting the spell?
I believe adventurers armory had ways to channel some spells with various tools (Burning hands with alchemist fire is the one I see used most often). Maybe hand him that to look over.