I know I'm late, as the 730 was due July, but this might help someone else in the future.
I would strongly advise you against trying to do his 730 yourself, unless you have a very strong command of Italian.
The 730 compilation manual is written using obscure legal speech and is quite hard to understand.
In legal speech common words can have a completely different meaning from what they normally do, and unless you're really good at feeling that "this is not quite right here" you may never notice.
I'm Italian and even I had a lot of trouble understanding some key passages of the manual, especially the section about life insurance.
One thing you could try, if your cousin has a PIN Dispositivo INPS (access code to log in to INPS' online services), is to log in to there and see if he has a 730 precompilato and it's acceptable to him (e.g. it doesn't say he should pay an additional €750,000,000 in taxes for some reason).
This is a pre-compiled 730 module that the Italian IRS gives you, and if you accept it as is you're exempt from any fiscal checks and any repercussions even if it's wrong.