Well, I'm (obviously) new, and pretty late to the game, considering how much of a necro post this is, but, considering how there doesn't seem to have been a definitive resolution on this, I thought I'd add in my two cents.
Frankly, the answer is staring us right in the face, simply based on how Dual Crowbars and Jack Handle actually work.
Strictly written, there's nothing saying that Jack Handle and Dual Crowbars wouldn't trigger on themselves ad infinitum. Jack Handle would react to each instance of it applying damage, turning Mr. Fixer into a zen Koan in which he hits all non-heroes for an infinite amount of damage, and yet never so much as crushes a tumbleweed. Dual Crowbars, on the other hand, would turn into an infinitely escalating frenzy of Half-Life-style violence until there is only one man standing.
It's obvious to everyone (I hope) that that's not how they're supposed to work. What then? The answer, of course, is typed damage.
On the one hand, we have the elements, which is the normal definition of type. On the other, though, we have the "special" types, such as irreducible. Mr. Fixer works on that basis, then. Jack Handle and Crowbars trigger not when Mr. Fixer deal any damage; only when they deal non-Jack Handle/Crowbar damage. Something to the effect of:
When Mr. Fixer would deal non-Jack Handle damage to a target, he instead deals that amount and type of Jack Handle damage to all non-hero targets.
When Mr. Fixer deals non-Dual Crowbar damage to a target, he deals that amount and type of Dual Crowbar damage to another target.
With that simple, implicit, commonsense fix, Friendly Fire works exactly as intended, without any need for rewording:
Mr. Fixer hits a target while using Jack Handle. Jack handle repalces his initial hit with Jack Handle damage of the same amount and type to all non-hero targets. Setback can use friendly fire to also take 2 Jack Handle damage (for each hit, no less).
Mr. Fixer hits a non-hero target while using Dual Crowbars. Setback can use friendly fire to also take 2 damage. Mr. Fixer's hit against the initial target and his hit against setback each trigger Dual Crowbars. But Setback using friendly fire on either of the crowbar reactions won't trigger the crowbars again. A little shennaniganey, but a face full of crowbar earns a little shennanigans, in my book.