Are they boondockers recharging off of a generator or pedestal campers? If it were me, and they are boondockers, I'd look for the Progressive Dynamics model that puts out 14.8V in bulk charge mode, not the 14.4V model, assumming they are using regular vented lead acid batteries.
If they have AGM batteries in their future, that don't corrode all your battery wiring and terminals, then I would go ahead with the regular 14.4V Progressive Dynamics model.
The exact chemistry of the batteries they run should determine the bulk charging voltage of the charger. Flooded lead acid batteries, 14.8V in bulk mode, AGM batteries, 14.4V in bulk mode, so they don't gas off.
Any of these will run off the Honda Eu2000i. The 60 amp units are most commom, easiest to find, and won't bog down the Honda and run fine in Eco mode on that generator. Make certain your friend warms up the Honda in Eco mode for 2 minutes before subjecting the motor to the charging load, then plug it in.
In either version, the 60 amp model will be fine, I think. These are boondocking dry camping suggestions. I haven't a clue for pedestal campers, I think it's far less critical for them, just float at 13.5 to 13.8V so you don't boil off the electrolyte.
If your friend is cheap, set him up with a pair of those Megawatt power supply units, dial in the voltage on that tiny set screw, and leave it alone, just watch the voltage come up and the amps come down, then shut the generator off. Set one of them up for 14.8V for bulk charging, and the second one up for 13.8V for float charging, if they are on the pedestal.
Mexico wanderer could provide all your answers, if your friend really wants to do it right, but someone here p*ssed him off royally, so he's gone buh bye... maybe you can email him for a better response, I am sure he'd help your friend out, if your friend wants to pay attention and listen. What you will get from him is a very specific answer, but there are many,many variables to consider what is most important to your friend. Battery life, gasoline and generator expenses, and life, and on and on and on. It really depends on how an*l or lazy your friend is on conserving or wasting money, vs convenience.
If your buddy is a boondocker... Get a Megawatt S15-40 from the guy up the road from you, set it up for 14.8v for him for bulk charging mode, first thing in the morning. Run that first on the generator, then let his WTF CO do the absorption charging on the generator at 13.8v (huge waste of gas) or get enough solar panels later to absorption/float charge the rest of the day, once the generator and megawatt have done the heavy lifting.