Are they riding fixed?
judging from the photo, I would say probably not; most of them have a SA (or similar IGH) visible and/or a third cable for a shifter going to the handlebars. There was such a thing as a fixed gear SA IGH but it wasn't that common.
I believe Rogerzilla has/had one of those hubs, so perhaps he can tell us which type of pedal he prefers to use with it?
(The thing about "oh but you only produce power on the downstroke so you don't need clips" is a massive spherical cow fallacy. If you're riding at constant speed in a straight flat line at average cadence and moderate effort, flats will do you find. Everyone knows that, and no adult with an IQ above 5 has ever beloved they can double their power by pulling up on the pedals. It's all the times you aren't merely pootling along the flat that they're useful
You need to be clipped in for racing or for riding in a group - any situation where your foot coming off could be a problem or even dangerous.
You don't need it for riding fixed, or even for high cadence, but if you find it helfpul for those, or any other, situations that's great.
To answer the OPs question, I use speedplay for racing and flat pedals for everything else.
Reason for speedplay is to get midfoot cleat positioning, which I (along with lots of others) gravitated towards after getting into ultraracing - as it fixes achilles / ankle / lower leg pain, various types of foot pain and is just more comfortable for many people after riding for long hours / multiple days.
Reason for flat pedals is that speedplay are a pain to walk in so didn't work for commuting and aren't great for audax, ultra racing, offroad, etc.
If Speedplay were easy to walk in I would use them all the time as being clipped in feels better and having your feet in the right place is one less thing to worry about. However, if you are able to pedal in cricles reasonably fluidly and can avoid yanking your foot up too fast on the upstroke (it happens now and again on climbs), then flats work well enough, once you get (re-)used to them.
If I'd only ever ridden events up to PBP-legth then I would have stuck with what I used before which worked perfectly fine. That was Look (Deltas - never bothered to change as they worked) for racing and SPD / flat pedals (double sided - would use both sides during long events) for everything else.
I don't recommend what I use to anyone else as, like everyone else, I have arrived at it as a result of a journey, and I don't much care what anyone else uses, as they undoubtedly have their reasons. But happy to answer specific questions from people who have issues.
To conclude, I think that, unless your friend wants to ride really long distances, anything would do. If he then has a problem, he might need to change something to address it, but can't anticipate that until it happens.
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