My son just had a cv shaft replace on the passenger's side. Now he says the transmission is slipping. When the shaft is replaced does the transmission fluid need to be replaced or could they have messed up putting the shaft in?

Is it an automatic tranny? No, the tranny would not need a fluid change unless they removed it.. If they removed it and the car is POST 75k miles, the asbestos on the gears are gone, and they must have gotten very hot since the axle was replaced and now the clutch will fail..

Sounds like a coincidence to me.