There's multiple layers to this onion ...
THD isn't really the issue, marketing FUD aside ... you could have a non-inverter gen, and it could be throwing THD into your pond of house wiring, but that's like adding clean water to an already muddy pond. Almost every device in your house is throwing off THD, so it's there. There is no simple fix, but the good news is that most every device in your house is designed to eat power of any kind, and not hiccup.
Also, how often is one running a gen? I run my non-inverter gens about 1000 hours per year ... no problem from THD here. Most folks run it for less than 50 hours per year. Your friend sounds like the scenario might be getting closer to mine (makeup power when solar not cooperating).
Surges (from the grid, from lightning, from your own devices in the house) *are* the big problem. Luckily, the fix is easy here ... a whole-house SPD in your panel, SPD's at critical devices ... done.
HVAC units (with cheesy power control boards), special inverters (w/ low-end power consumption design) ... these could indeed be scenarios where you have to use an inverter-gen, especially if the choice is already made on these units (they are existing), and now you have to feed it some level of good power because, unlike every other electrical device in the world, it can't eat iffy power.
I would spend time really digging into the Victron or other equipment documentation, and seeing what range of THD they want to deal with, and then focus on the gen that gets you in that range, and also meets all "other" requirements (fuel, capacity, etc.) ... could be inverter-gen, could still be non-inverter gen, but at least it's more of an informed choice.
I don't have Victron (I have Magnum), so I can't speak to what Victron's requirements are ...
Hope this helps ...