He will probably need a larger alternator if he wants to avoid voltage drop under load. 1600W (assuming that's a continuous rating) means he'll be drawing quite a bit of current just to run that amp. Dual batteries will only help with reserve capacity, they won't do anything for voltage drop. Stay away from capacitors, they're a cheap and dirty band aid. A good single AGM battery capable of very fast charging and discharging (I recommend Odyssey Extremes) and a strong alternator combined with the big 3 wiring upgrade mentioned above is the recipe to follow.
