If a side of a dragon is dead, i.e 3 skulls.

Can an attack be made on the dead side, making the controlling player choose which side to tranfer the damage to, or is it an illegal action?

As far as I can tell from the dragon rules the section with 3 skulls would be inactive and the attacker could decide which section to resolve the attack. Hope that helps.

The attacker chooses the section being attacked when the attack passes through a section that shows 3 skulls.

A friend told me that when a section of a dragon is dead, you can not attack from this section to the anothers.

If an attack is made against an inactive section, the attacker chooses a different active section to resolve the attack against. The attack against that section must still be legal.

(You can't shoot through hindering terrain at an inactive wing of a VSD and choose to resolve it against a head or tail showing Stealth, for example.)

you can attack a dead side and then the attacker chooses which side actually takes the attack

Thanks guys

i have kind of thought that a sweeper of shockwaver should be able to hit the remaining three sides, but i think that it is still attacker choose one side. No official ruling though.

as you can only attack one side by the use of any range attack (line between the dots) your shockwave could only hit one side...if this one is already dead, proceed as written before and chose another.