Like Emmo said,
one thing to note as well, is that the Strongarm effect wont apply to mobs that cant be knocked back like Rift guardians and alot of big monster types. Also crowd control immunities can affect the uptime on Strongarms.
If your question was regarding Solo play I would say Magefist would provide the better overall value.
The Strongarm effect however is prefered to use in group, since it will buff the dmg of all party members done to those mobs.
This also depends on how much fire additves he have already since 20% is not a 20 boost anymore.
that's why "like Emmo said"... he should check his overall gear. I highly expect no matter which class and build that at least there are some additive dmg bonuses already available equal to 40% (so same existing value like like fire neck + bracer), hence suggesting Magefist