I once had a neglected plant given to me do the same thing...1 leaf leaves for awhile. Didn't even look like weed I kept it under 24/0 light and watered it for a few weeks and it eventually went back to normal leaves...I didn't flower it until it was pretty much back to normal tho. Mine turned out OK, but you'll need to watch them for pollen sacks as they may herm on you from all the stress. They were free, so anything they produce should be a bonus, right?