How bad is he abusing the dog (apart of keeping him prisoner on the roof) ?
Kicking with his leg ? With a "palo" ?
What kind of breed is it ?
I would not go ahead and talk with the guy as a foreigner (he probably would answer "que carajo te importa ?", "porque no volves a tu pais ?"), furthermore it seems quite evident that this guy would pass his nerves on the dog (from what I could feel based on your description).
At worst, the guy will beat the hell out of the poor animal, at best he will release it in the street... Or things will remain unchanged.
What I would do :
- videotape the guy on the roof (it's legal, it would not be in the guy's patio).
Preferably videotape the guy from a window which is not yours (window in the building's staircase ? something similar). If you have to videotape it from your home, don't include details that could identify you (like curtains, ...).
- talk with some of your Argentinean neighbours to see what can be done
- then, eventually, have a group of your neighbours file a denuncia at the PF : At worst, they'll do nothing, at best they'll go speak with the guy but then, what will happen to the dog ?
Last : the picket thing is not a bad idea, it certainly will be something he will remember.
The idea here would be to have the guy kicked out, fair counterpart.