Just to let you know that I am not a professional but I do think you can cut it down to a few metres now and just take the excess and toss it away. It is probably a montana or could be an everygreen group. Whichever group it is, it most certainly belongs to group 1 pruning. These are clematis that grow 20 to 40 metres and do not require pruning except to just tidy up OR if it has outgrown it's space. It sounds as though this woman wants it pruned and if it has already flowered (if it's a montana it has already) then it can be pruned down now to a few metres. I would just double check to make absolutely sure this is what she wants because they really are beautiful in maturity but then again, they grow extremely fast also.
If I were you I would ask the same question in the Clematis forum here at the Gardenweb. There are many people there that are professional in this area and many even are cultivating their own hybrids. Goodluck flower powergirl.
Happy Gardening
Clemmybug