I tested this. In this case, the Will-O-Wisp would be redirected to its user rather than absorbed by Flash Fire, making Magic Coat take precedence over Flash Fire. However, if the original user of Will-O-Wisp has Flash Fire, it could absorb the attack as normal with that Ability: "Whenever a Fire-type attack by another Pokémon targets bearer..." since the redirected attack is an attack by Porygon2 against Rotom.
See my timing page, section "Targeting"
If Magic Coat is in effect for the target, or the target has Magic Bounce,
when certain moves are used, that target uses that attack instead. (See attack
description for Magic Coat for details. If this effect happens, effects that trigger
whenever that target is targeted by an attack don't happen.)