Eric should be cautious not to overtrain; swimming 10,000 yards several times a week may do him more harm than good in the long run. If he's determined to cover 50,000 yards a week, I would suggest swimming 8,000 yards, five times a week, focusing more on speed and pace work, with a 10,000-yard workout on the weekend followed by a day off.
Getting back to the goal at hand, which is to put Eric under 19 minutes for the 1650, here is a sample workout that manages to cover 6,000 yards while giving him an opportunity to try repeating his goal pace per 100 at 1:09. Even if he chooses to keep swimming long-distance workouts, he ought to incorporate elements of the main set below into his swims.
Warm-Up
600 freestyle200 backstroke
200 breaststroke
10x 50s @ 5 seconds rest, descending effort 1-5 and 6-10
Main Set
500 freestyle5 x 100s @ 5 seconds rest, faster than usual pace
500 freestyle
5 x 100s @ 10 seconds rest, aim for 1:09s
500 freestyle
5 x 100s @ 15 seconds rest, aim for 1:08s
1,000 pull, recovery
(25 easy, 25 sprint) x 10
Total: 6,000
This workout is a basic yardage-covering workout, with an emphasis on repeating 100 yards at a consistently faster time than one normally swims in automatic mode. It covers distance without sacrificing quality, and the repetitive nature of the main set provides ample opportunity to swim fast and maintain race pace.
The sprint 25s at the end of the workout are meant to further jolt the swimmer into sprint-form. Over time, such sprint drills condition an athlete to swim faster for distances and this eventually results in swimming faster for longer periods.
For example, if your normal pace is 1:11 per 100, it makes sense that one way to drop that time is to drop your sprint time from 1:02 to 1:00. The easier it is to swim a sprint 100 at 1:00, the more feasible it will be to repeat several 100s at 1:09 pace.
Swimming 10,000 yards per workout is a great way to prepare for swimming extreme distances. To put Eric's dedication into perspective, when I trained for the 13-mile Swim Around Key West, the most I ever covered in one session was 15,000 meters. I only did this once a week for a month before the race (and I subsequently shortened my training the following season to a maximum daily 10,000 meters).
Generally, I recommend the two-thirds rule to swimmers who ask how much they should train for extreme endurance swim events. For a 15-mile swim, it is only necessary to feel comfortable training 10 miles at least once before your event (likewise, when I have trained for running marathons, I have usually only ramped up to an 18-mile training run as my longest distance before the race).
Unlike such above endurance events where the goal is to finish, a shorter race like the 1,650 requires focused training that emphasizes a different kind of endurance: the consistent pace held per 100 yards.
I am willing to bet that Eric would still be clocking in more distance than his masters competitors if he committed to a daily 4,000 to 6,000 meters. The important thing for him to remember if he wants to improve his time is that learning to swim fast is more important than swimming long and slow.