Hello!

[cornixt]

All it takes is one player to be having a bad day and it screws up all the other characters.

[Daedalus]

The NA instruction booklet does rather imply that it is only possible to attack a monster rather than forbid in words to attack another hero:
p. 13: "As a Hero you may attack any monster that you are adjacent to."

Vice versa:
p. 22: "A monster may attack any Hero that it is adjacent to."

For spells, the cards state who the spell can be cast on.
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Let me know if I succefully did the Daedalus here.

Indeed, the Daedalus has been done successfully. However, I'm left wanting more...allow me to do myself:

Under What Makes Hero Quest Unique from page 5 of the NA Instruction Booklet:

• The Heroes work together to defeat Zargon and his forces of Chaos. Individual winning is not the goal. United the Heroes stand. Divided they fall.

This is echoed on page 10 at the end of the first paragraph of A GLIMPSE INTO THE WORLD OF HEROQUEST:

...Your very survival depends on your ability to work together, for no true Hero stands alone.

Combine that with what KK posted about how a Hero attacks and the fact that NA spells specifically.attack or negatively effect monsters only. Taken as a whole, it is fairly certain that Heroes aren't intended to attack other Heroes in the NA game.

That being said, it is possible for a Hero to attack another Hero under Zargon's control due to the Chaos spell Command, as that Hero is moved by Zargon and may attack as a monster. Similarly, Quest Notes can "...detail the unique situations the Hero players will have to contend with." That pretty much allows for anything, including Heroes attacking each other.