Question: what to do if someone has a seizure while sitting?
Answer: Put the brakes on, to stop the chair from moving.
Let them remain seated in the chair during the seizure (unless they have a care plan which says to move them). ... 
If they have a seatbelt or harness on, leave it fastened.
If they don't have a seatbelt or harness, support them gently, so they don't fall out of the chair.