Here's one for the professionals?

Being spring / friction washer are long time in compressed position I am assuming these should be replace when stripping down parts. Also am I correct that bolts/nuts securing for example timing belt brackets have a set tightening torque and if not met, ie: lose could undo itself with vibration of engine. Reason my daughters cars engine is shot £2,000 to repair because the timing belt tension bracket bolt came out.

The garage she bought the vehicle from and who changed the belt before she purchased it refuses to accept liability even though they stamped the service book that the belt has been replaced and has a receipt to say this.
Many thanks in anticipation
Bob.