Clematis wilt is rarer than you would think. Usual reason for Clematis to collapse is damage at the base either mechanical (hitting the stem with something) or mollusc damage. snails love to strip the bark from the stems.
This kind of damage makes the stems droop from the top down. The fungal infection of Clematis Wilt causes the stems to die from the base upwards. Also if a stem is cut there should be a purple stain inside.
I would look for damage to the base of the stems myself.

Thanks Owd. Could very well be mollusc damage as my friend hasn't been using slug pellets for the past couple of years because she has a cat.

If it is clematis wilt it should regrow next year and be fine. Unless your friends cat eats slugs and snails advise her to use 'Slug Clear' as an alternative to slug pellets. It is applied as a liquid by adding a capful to your watering can.