It's SUPER easy to set up an SEP-IRA with Vanguard! I have one myself, that I set up for a very small side business. They even have a calculator on their website that will tell you exactly how much you can contribute based on your schedule C info.
That is the one downside I see with an SEP-IRA...your contribution amount is entirely based on your earnings/profit, so it's hard to estimate in advance (IMO). I always wait until I do my taxes and then make my contribution AFTER I've calculated everything. Just like with a regular IRA, you can contribute any time up to the April 15 filing deadline. I'm not sure how you would do automated contributions. Maybe his accountant would be able to tell you.
It's been a couple of years, but I'm pretty sure I was able to do the entire set up online, which was great. I did it on a laptop, though, so I have no idea how easy it would/would not be on a phone.
ETA: Once you have the set up done, the Vanguard app is very easy to use. If you can be there to help the set up, I think he would be able to handle it just fine from his smartphone.
He definitely needs to be using some kind of tax-deferred investment vehicle to lower his tax bill. It can make a HUGE difference. Personally, I think the SEP-IRA is the easiest to use for a single person business. Combine that with an automatic contribution to a traditional IRA (or Roth depending on income limits), and you are well on your way to significant savings.