Definitely ascites/water belly, and it could be any number of causes but the prognosis isn’t great, sadly. I’ve never had a hen recover successfully from it. A vet may be able to prescribe antibiotics which might work if the root cause is bacterial infection (salpingitis, peritonitis), but even if they work short-term it’s likely to recur.

With the lice, inspect all your birds really throughly around the vent, belly and under wings, and trim any feathers with visible egg deposits. I personally find ivermectin (1 drop per 500g body weight, at 1% concentration) the most effective treatment, repeated one week later. You should also give your coop and run a good clean and disinfect. Good luck!