I am not sure of the year of your MB but Also with the flex disc is the carrier bearing. If he pulled the bearing it would do that every single time due to the weight. I would think the shaft wasnt balanced if it was that bad. Also the flex disks maybe incorrect for the vehicle. If he got it with larger holes or didnt make sure the spacers were in it could cause all that vibration.

Like everyone has said here go back to the original problem. Its most likely there.

Like if he changed motor mount or something and you have a decent missed it would feel crazy.