GMC Repair: bad clutch and transmission, pontiac gto, clutch

Question
Can clutch cause the transmission to go bad? or vice versa?
My son has a 06 Pontiac GTO (manual)35000 miles.
The transmission start slipping or missing gears.  He took it into the local gm shop and they told him that it was probably the clutch. (not covered under warranty) After he got it back it was doing the same thing.  So they replace the transmission (said it was to bad to repair). They told him that because the clutch was in such bad condition that it caused the transmission to go out. Is this even possible?

Answer
yes think about it know you have onewheel with teeth and the other with teeth every time it slipps the other teeth will slip and vice versa sounds like they are right Its caused from not shifting properlyand holding down clutch pedal to long while after shifting