Actually the Boondocker is a great value for 35 amps and will charge just fine. You can add some wire leads and clamps if it must be portable. Although not designed to be out in the weather it would have to stay under the shell.
Does he charge with a generator? How big? Need 2000 watts to power a large charger to get a fast charge into 2 batteries. A 1000 watt would power maybe 35/40 amps max.
For plugged in at home I recommend 10 amps minimum. Do you want a sealed unit to set in the mud and let run overnight in the rain?
Here are a few to look at:
https://www.batterystuff.com/battery-chargers/12-volt/
