Hey,
AimIK doesn't do any networking on it's own, you'll just have to sync the target position over to the remote client(s).
With AimIK you have 2 ways - sync aimIK.solver.IKPosition using on PhotonSerializeView or assign an already synced Transform as the IK target. Keep in mind that if you have the target slot assigned, it will overwrite aimIK.solver.IKPosition before each update, so if you synced IKPosition, but had an unsynced Target transform, it would not move.