I have a certain view of parenting and family, and a certain visualizatin of this situation (as we all do), and I really don't think the grandparents did anything wrong.
1) Yes, a heads-up to the parents would be Best Practices. The grandparents assumed that the parents were okay with it, rather than explicitly discussing it - not Good Form. However, let's not lose sight of the fact that their assumption was, in fact, correct. The parents did NOT have a problem with it. Perhaps this speaks to how well they know and understand their son and daughter in law, or a certain level of shared attitudes/values in the family. Not every family has that, but apparently this family does.
2) I think the hyperbole about "being electrocuted" is overdone. Doing "apprentice electrician" work does not generally involve live current - he was probably bending conduit and stringing unconnected wire, handing off tools, etc. Patching a roof can be done from inside or outside, though an 11-12 year old kid on a roof with supervision does not strike me as radically more dangerous than anything else an 11-12 year old kid would be likely to do WITHOUT supervision.
3) There is a big parenting philosophy split here, as well. If a 9-12 year old kid is living with his grandparents for the summer, and Grandma and Grandpa are going over to work on their new house today, no. I don't think the kid is entitled to say "You have to stay home and watch me play video games, or you have to take me to the zoo, because I don't feel like helping." Neither do I think he is entitled to sit around on his kiester while his what? 60-70 year old? grandparents are working. That dog won't hunt in my family. This is not child abuse or exploitation, this is family respect, and grownups being the leaders and decisionmakers of what the family is doing. So it should be. (in my view). I really, really doubt that the kid's mom and dad would have sent him to his grandparents' for the whole summer if it were some kind of Dickensian workhouse. The dad grew up with these folks, the mom knows them and has heard all the dad's stories - and they chose the grandparents as safe, appropriate and caring people to keep their son for three months.
I do think the boy has the right to say, "I'm scared to go up on the roof" or "this sledgehammer is too heavy for me," etc, - he has some voice in what tasks are appropriate or that he is able to do. I also think it would be good if the grandparents paid him. However, surely a tween is old enough to understand that the nice house he lives in with Grandma and Grandpa, and the nice food on the table, and his nice Christmas presents, etc - all exist because of this work. Enjoying the fruits of your labors is one of the greatest and most reliable pleasures in life, and the "fruits" are not always financial - sometimes they are tangible and direct. I don't think its ever to early to start drawing those connections.