My rationale was that, if a person casts Invisibility, anything they're wearing or carrying goes invisible with them. That could include another person, so long as they're carried. They could even pick up such a person after they're Invisible, and conceal them as well. They just need to cover them with a cloak or similar item.
So the caster got a free ride but, unlike the Dragon, was not actually the target of the spell. If the Dragon had cast something like Scorching Ray, each image would have had fiery beams coming out, all focused on the same target(s). But the rider didn't really have that protection, so only one set of rays came out, all from an identifiable source.
I really need to get better at staging high level battles, by the way. But that's another story.
