He says that he used to run a lot when he was younger, but got busy with life, and now regrets stopping. He is about 50, and still wants to do ultramarathons someday. Right now, he is sitting at about a 13 minute mile. He's tried sticking to a typical couch-to-10k program in the past, but it hasn't worked out great. What would be the best way to proceed with his goal of running a half marathon by September?
Can he still sprint? I'd start training him for the 100 and 200. Ultras are for the weak who don't want to feel the pain. The sprints are where it's at.
How much weight has he gained since he "used to run a lot"? I'm guessing it's a decent amount, and if so, tell him not worry about running right away. Just do enough activity, walking, biking, a little running is fine, along with a good diet to start losing weight. As the weight decreases he can add more running. But for the next 6-8 weeks the goal should just be activity, whether that's counting steps or "time on feet" each day/week.
The problem with only running when people are older and out of shape, is they simply can't get in enough miles to get the exercise benefits their body needs.
He's not ready for structured training, not even couch to 10K. He needs to be jog/walking until he can jog steadily for 2-3 miles, and doing it 3-4 days a week until he can do it 4-5 days a week, eventually aiming for 5-6 days a week.
It feels slow. It feels dumb. But it's the only way to get back to consistent training without getting injured. The pace doesn't matter. Step 1 is to work back to getting out the door nearly every day, and nothing else can happen until he's been doing it for a while.
The big enemy at this point is injury. He's got to build up gradually to avoid all the muscle, joint, bone and ligament injuries that are just waiting to take over if he gets overambitious.
What's his weight like? That plays a big factor. Adding a basic weight routine a few times a week can help too.