I've noticed older leaves on Java Ferns tend to have this browning problem where they get brown patches or form little holes and slowly become "skeletons" and whatnot (and this is NOT due to a K deficiency). I just let it go, as it seems to happen to most people, and eventually old leaves will fall off, and I just let them float in my tank until they eventually sink and turn into leaf litter for my shrimp. Usually the leaves stay green and float for like 2 weeks at least, and during that time, several little plantlets will grow off it even though the leaf is unattached to any rhizome or roots.
I have all these little plantlets right now that I don't know what to do with. I'm considering setting up a Zapins style emersed set-up for them to grow up, and maybe attaching some to driftwood -- either now, or in the future.