This is a bit extreme, and I'm sure the owner will forgive me, but this is the result of no maintenance. Poor maintenance or the wrong maintenance will have similar results - but it may take a bit longer to materialise as defects. Basically the cord holding the sash window weights have broken (as they do) and have sat against the timber facing of this dormer window until the timber decayed sufficiently for the 'weight' of the weights to force their way through the rotted timber. I know most of you will be adamant that you would have carried out some maintenance before this stage but what if there is so much to do that there isn't a clear place to start? It can become totally overwhealming and lead to inaction.
The solution? Learn how to maintain - and get it done before things cost more to repair.