The plastic will fall apart after prolonged overheating. Far more likely to have been caused by inadequate tightening. With any half decent fitting the screw slot will get damaged or the head shear off first if you did overtighten - not the thing break. And that includes the no name fittings you get in the sheds.
Not tightening connections properly is very common. I always tighten them up 'till they groan. And have never ever broken a screw or fitting. Your friend is not as knowledgeable as he claims, and I'd get his work checked by a decent electrician.