When my garden was still a dump we also had several clumps of ferns almost a decade old. They never got cut back in winter or spring, the old foliage just rotted away without any problems for the plant itself. When I dug them out during the rescape of the garden last year I kept a few and planted them in a tin barrel. It was midsummer and I cut ALL the foliage away. They rebounced within two weeks and grew on 'till the first frost. After that I cut all the dead frons off. We'll see how that works when they begin again. So far it doesn't seem to have hurt them because the still unfolded frons are nice and firm.
