Yes you are correct, it's been years, so I'm a rookie too.
I was just trying to offer a way that didn't require
too much effort or intensive labor costs.
I figure a full rafter would involve a lot of extra
effort to run another 2x6x16 full length,
down the side of the cracked/split one.
The attic might be a pretty tight fit to get a 16' board up there with out removing the roof.
Not to mention the opening and
clearance at the attic's access point (hatch?).
Of course the best thing to do I guess, is have the neighbor file a claim
and get a pro to do it.