What year and engine exactly does it have?
What condition is the friction plate when removed do you know?
Have they checked for contamination at all?
Ok i guess first of all the garage would need to asertain exactly what kind of oil it is? if its engine oil..then obviously they would need to check for crank seal behind the flywheel for leaks.they would need to remove the flywheel to inspect that..of course the flywheel should always be inspected for issues or replaced even..ie: scoring , blueing,heat damage , and being a dmf on the DTI'S for excessive movement can all play a part in premature failure of a clutch....have they checked for excessive oil leaks on the vacuum pump on the side of the cylinder head.there is seal there that can leak...also any leaks around the sump gasket closest to the gearbox. that maybe seeping into the bellhousing....all needs to be checked....they only other seals is either on the gearbox input seal..though normally you'll notice alot of damp or oil in the area once the slave is removed....There isnt really much else could contaminate the clutch itself bar those possible areas as above...But of course the quality of the clutch/flywheel needs to be taken into account..and your garage should detail exactly the branding they are putting in....