Actually, the BEST training plan is to hook up with your local tri club. There is a plethora of knowledge there as well as training buddies. This is key as he really needs to get out of the pool a few times and into open water - requiring support of others. Swimming the Potomac will be a lot different than the lanes of a pool. Usually you can transition easily from a pool to open water on shorter formats, but when you start approaching a mile, with a u-turn you really need to experience a few open water swims.
Now as a coach and a triathlete, I relay on a three sets of plan for ideas. The first is Matt Fitzgerald. His book, Triathlete Magazines Essential Triathlon Training Plans is a book with training plans all the way up to IM. I base my training on his plans
first because I have them downloaded and ready for use in Training Peaks, second because you can vary the training intensity based on your current life style, base condition and in my world, it is in a training log and creates a daily email. Another good set of plans is from Joe Friels Training Bible and also some from from Gail Bernhardt.
The big difference between running and triathlons is that not very much is free in the tri world. And most plans you find for free are really just outlines of a real plan that someone lifted. Also, most tri plans will have some competitions included in the training plan. This a needed training weekend so that you get a feel for the mass swim, transitions under fire and putting all three legs together under a clock.
So
Hook up with a tri club.
Get some open water time
Get a good training reference book
Use sprint tris in the training period.
Have fun