Live plants are great and Bettas love them, java fern is a good choice and usually does well in lower light condition with just about any water. Usually the light that came with the tank is all that is needed to grow java fern, however, the better the light the fast better it will grow-they usually will even grow and do fine in tanks without light that only receive in-direct light.
Java fern does best when tied to something like wood, rock or any decoration-you don't want to bury the rhizome in the substrate or it will rot and die
With any plants it is best to leave the plant in the tank with water changes, however, in unfiltered tanks with just java fern and no other species of plant-you can usually remove the plant and whatever it is attached to for the 100% water changes if needed-sometimes java fern will send secondary roots down into the substrate and if they do it is best not to disturb them-but if you do it is usually not harmful to the tough java fern. Even the brown leaf of a java fern can contain spores that can make a new baby fern...so don't throw it out......
If you have other plants like stem plants rooted-it is best not to disturb them even in unfiltered tank that you would normally do 100% water changes-with added stem plants-the 100% water changes are not needed because the active growth of the stem plant will help in keeping water quality good by using waste, ammonia, nitrate and DOC's for plant food...
key word-Active growth of fast growing stem plants, although, java fern is a good plant it is a slow grower generally and uses less of the waste than fast growing stem plants when used for water quality.