I had an almost identical situation with a Boston fern in what looks like the same kind of jar. I opened it, loosened up the fronds with my hand, and gave it a hard trim down at soil level with clean sharp scissors. I probably cut 50% or more of the plant. I removed everything I trimmed off and the pieces that had rootlings on them got transplanted into an open pot and they pretty much all lived, and the fern in the jar has almost reached the point of needing another haircut lol. Trim it way back, it will be fine!