Shield of Missile Attraction: does one has to change the attack roll?

I got interested into the cursed item Shield of Missile Attraction, but I have some doubts on how it works. Consider the following scenario:

  • Thurin the Dwarven fighter is walking along the city street, it wears full plate armor and has this cursed shield. Total AC 18.
  • Dalena the Elven mage is just some steps beyond Thurin (hence inside the 10 feet radius). She has a total AC of 15.
  • Greg the thief is planning to kill the mage, from the roof, since she killed his beloved one. He rolls for attack with his crossbow: 16.

Since the fighter has the cursed shield it becomes the target of Greg’s attack. My question is

  • does Greg have to use the original attack roll (16, in this case) because the shield changes only the target? In this case, the attack misses and Thurin does not get hurt.
  • does Greg have to roll another die for the attack? In this case the dwarf may get hit.

The first one seems to be more fair, since the attacker actually does not know that the dart will be deviated towards the owner of the shield.