The parking garage is likely the most liable for this collision.

Line of sight is blocked leaving the garage for both drivers and sidewalk users. The garage has not compensated for that by either putting up a mirror and/or visual/audio warning system that a car is leaving the garage.

Boston may prohibit bicycles from sidewalks, but many cities/states do not or just limit the area to the center of downtown. Many cops would ticket a scooter if they used the road.

Sounds like the city only has rules and not laws/ordinances for the scooters. There will likely be several legal battles between cities and scooter companies because of this.

Scooter rider may have some liability if the sidewalk has a 10 mph limit and it can be proven the scooter was going 15 mph or over. Lots of runners travel at 12 mph.