Aggemam -
Careful here. "Bonus damage" is damage like that from sneak attack or flaming weapons (which IIRC have their own crit rules anyway).
When rolling to "confirm" a critical, you use all of the modifiers of the previous attack. The weapon focus factors in on the critical roll
Critical hit damage is usually double normal damage, which means rolling damage twice, just as if the attacker had hit the defender two times. (Any bonus damage dice, such as that from a rogues sneak attack, are not rolled multiple times, but added to the total at the end of the calculation)
"normal damage" in this case is 1d8+5
1d8 from the bow +4 due to str mod(legal because of mighty bow), +1 due to PBS, and +1 due to favored enemy.
This is presuming the ranger was within 30 feet when he scored the critical so that he gets the favored enemy bonus to damage. As the comp longbow has a x3 crit...
the crit damage is
(1d8 +6)x3
or
3d8 +18
If your player just rolled a single d8, added the +5, and tripled it - he did it wrong. You actually HAVE to roll the damage three times. If you don't - you don't get the right damage curve.
42 points is the absolute maximum possible damage given this scenario. The actual damage curve is 18-42 points. Even at the other extreme of the crit damage, this shaman is at -8 hp... and would be at -9 before the ranger could get an action to check the gnoll's vital signs. "Shred" is the right word.
If your player managed to get max damage on a crit using a greataxe (2 handed weapon) against the same foe at 1st level; the maximum damage is a whopping 54 points - enough to call for a death from massive damage check.
It is not impossible for low level characters to "dish out" this much damage on sucessful critical hits. That is part of the reason why lower-level combat is so dangerous.
Keen Scimitars in the hands of a charater with the improved critical feat are also VERY deadly - you can expect a crit rate of 45% - but there are other combinations.
Edit - Thx Thanee - post corrected for the math error.
Aggemam - when I read your post, I thought you were refering to the "bonus damage" from the strength score, PBS and the ranger's ability - and indicating that this was not multiplied.