So, we are all in agreement that a lone character is considered a unit for game purposes. And, if we ignore the Resurrecting Fallen Warriors rule for just a moment, it would be a valid target for IoN, as it is a unit.
Okay. So, since it is a valid target, let's look at Resurrecting Fallen Warriors, which is an addendum to 'spells and items that restore lost wounds'. As IoN is a spell, it is affected by the Resurrecting Fallen Warriors rule. As many have said, "Unless specifically stated otherwise, spells or items cannot heal characters or their mounts", would prevent characters from being healed by IoN, because IoN doesn't specifically say so.
However, it DOES specifically say earlier in the Resurrecting Fallen Warriors rule that Lone Characters can be affected by spells and items that restore lost wounds (IoN). So, yes, IoN doesn't specifically state it, but the Resurrecting Fallen Warriors rule specifically states it.
Furthermore, lone characters with mounts are still lone character units. So, any wound they regain from IoN could be applied to either the mount OR the character, as IoN says that the unit regains a wound.