"No problem, there really is no neutral on 120v single phase inverters we just call it that"
- If he said this, it leads me to believe that he doesn't know what he's talking about, or at least doesn't get the lingo.
The problem with inverters bonding the neutral to ground is that there are (3) general possibilities of how inverters are used:
1 - Portable, with no ground (it doesn't matter if neutral is tied to ground if it doesn't exist)
2 - Portable, with ground (there's no panel, so neutral should be tied to ground in the inverter)
3 - hard-wired, with ground (there's a panel, so the tie should be in there, and the inverter shouldn't do it)
The problem is that inverter manufacturers like to sell an inverter that works for all possibilities, but they can't satisfy all criteria unless they allow you to change the bonding status. The authorities didn't like that, so they made it be internal. For inverter manufacturers, it's safer for them to bond it if the inverter can be portable, because it's better to have 2 bonds than none.
The practice of having only one route from neutral to ground is more about when you have a sub-panel pretty far from the main panel, but not far enough to have its own separate ground. In that case, if you bond in the sub-panel, your ground wire back to the main panel will then act as a parallel neutral conductor; since it's often bare, and not expected to be carrying current, that's a problem.
Assuming that your inverter is in very close proximity to the main panel, I would just wire it like he says, and make sure to use a ground wire from the inverter to the panel that has green insulation the whole length, so if it does end up carrying neutral current (and it will), it's not such a big deal. I believe that the AHJ should be okay with this, but you can never tell; often, they don't want to admit when they don't understand something, and will disallow it if it is not verbatim with the code.
There is another potential problem with your setup. I assume the main panel is 120/240, and you plan on putting in some kind of switch to be able to use both legs in the main panel, so all 120V circuits will be active. If so, you need to make sure there aren't any multi-wire branch circuits, or that neutral can become overloaded without tripping the breaker(s). In a multi-wire branch circuit, one 4-wire cable has leg 1 hot, leg 2 hot, (1) neutral, and ground. Normally, the two hot legs are out-of-phase, so the neutral will only see the difference between the two circuits. However, if they are in phase, it will see the addition, which may be up to double what it is rated for. You also need to make doubly-sure that it's not possible for that switch to be active at the same time as the grid; I'm pretty sure you will have more trouble with the AHJ on this one, and I'm not aware of off-the-shelf hardware that will do this.
