What is the process to request wheelchair assistance or staff assisting for my friend( impaired mobility ) at the metro station?
If the station is wheelchair accessible (see the article/map on wheelchair accessible Paris train stations) it will actually be the train conductor (in the lead car) that will help you board/alight the train using a portable ramp, not the ticket window staff. And this is for RER trains as opposed to Metro trains.
To signal for help from RER train staff, when on the platform awaiting arrival of the train, wait on the platform near where the head of the train will be when in station / at the platform. For this you must know which direction the train is traveling on your platform. This is sometimes marked on the platform by overhead signs signaling where the train should stop, both for the lead car and last car, when parked at the platform. The front of the train will be marked as devant (front) or arrière (back) of train. You want to be waiting at the front of the train where you can wave to the conductor and point to the wheelchair, which will make it obvious what you are wanting. If you are not sure which direction the train is traveling and where the head car will park, let a train pass to give you a sense of where the lead car will be when at station, then aim to board the following train after signaling for assistance from the conductor.
Metro trains that are wheelchair accessible won't have any significant gap between the platform and the train itself so the entire path from street entrance, ticket gates, tunnels/elevators to platforms and boarding should all be wheelchair accessible without need of assistance.