Overhand Chop, and the related Backswing ability, both appear on p.108 of the Advanced Player's Guide. They are abilities listed under the Two-Handed Fighter Archetype, and are not feats. Neither has a "Benefit" or "Normal" line.
Perhaps some players are thinking these are some old feats with similar names, but they are not at all the same thing. Here are the complete descriptions of both abilities (I have not edited them in any way, or left out any additional text).
Overhand Chop says:
"At 3rd level, when a two-handed
fighter makes a single attack (with the attack action or a
charge) with a two-handed weapon, he adds double his
Strength bonus on damage rolls. This ability replaces
armor training 1."
At 7th level, when a two-handed
f ighter makes a full attack with a two-handed weapon,
he adds double his Strength bonus on damage rolls for
all attacks after the first. This ability replaces armor
Under RAW, it is crystal clear that the original player, with an 18 Strength, should gain +14 damage when using Overhand Chop, and +8 damage for all swings past the first when making a full attack. He is actually shorting himself 2 damage by only applying 14 damage (I think he forgot that he normally gains +6 damage from his 18 Strength when using a 2-handed weapon).
The Overhand Chop and Backswing damage bonuses are totally unrelated to the 1.5 Strength bonus for using a 2-handed weapon (indeed, they replace the Fighter's class feature Armor Training 1 and 2, which has nothing whatsoever to do with damage otherwise). The player is giving up Armor Training to gain damage, which is not surprising since he chose the Two-Handed Fighter Archetype.
Overhand Chop 18 Strength
+6 for using his 2-handed weapon
+8 more for Overhand Chop
Backswing 18 Strength
+6 for using his 2-handed weapon on all attacks in a Full Attack
+0 from Backswing for the first attack
+8 from Backswing for all attacks after the first attack