Sometimes,during healing of any incision,there may be formation of swelling and it will be seen as lump.
the same happened to your husband in the scrotal wall.It's not a big problem.
There will be multiple layers in the scrotal wall.All of them have to be incised before reaching the Hydrocele
sac.Once the sac is opened and repaired after letting out the fluid,these layers will be closed by sutures.
Because of the suture material also,there may be chances of swelling in the incision site/suture line.
so,it can happen to any Surgical procedure ,even Hydrocele also.
But,nothing to worry.
All the best