If she hit the rock with the tire, the wheel would bend before the suspension is damaged. If the tire survived with only a bubble, I am very skeptical that any suspension damage could have occurred. Perhaps you are looking at prior damage?

My spouse made a glancing hit on a good size rock at 60mph in our Honda Odyssey. It destroyed the tire sidewall and bent the alloy rim. The rock was thrown against the undercarriage of the car, where it scratched things up and dented the exhaust a little bit. But the suspension was not damaged. The alignment wasn't even thrown off.

So I share your skepticism of the Subie dealers diagnosis. Anything the tire survives just can't be that bad.