If he's willing to work with jpgs and wishes to use a mobile device, I would suggest just download Photoshop and/or Lightroom. Both are free, or should I say have very adequate free versions, especially if only posting to the forum.
I employ Photoscape (free ware) but I'm not sure if it mixes well with Mac, as I've always been 'Windows'.........
Yes, for forum posts, but the man has already said he prefers to shoot raw and jpg and is why I felt it reasonable to address this.
Wow, Preview is so cumbersome, and s-o-o-o-o SLOW. It takes over a minute just to launch. Spinning beachball galore! And yes, I do NOT trust jpegs as my "go to" format. It's okay for some junk, but I'm a guy who spent his entire career in photography using the slowest, finest grained films in the world. Just as in coins, I am a "quality snob". I. Do. Not. Make. Compromises. And I have the awards to reflect that.
But you just did make a compromise when you posted a lowly iPhone jpeg selfie upthread. Why? Because it was the right tool for the task. I use a pro grade version of PhotoShop Creative Suite for my work, but for a quick resize and tweak of a pic to post on a forum, Preview works just as well (and last time I checked, nobody is passing out awards for pics shared with forum friends). If Preview takes over a minute to load on your computer, you might want to have your machine checked out. On every Mac I've ever used, including the two I'm running now, Preview loads instantaneously. Which it should, given that it's under 40MB. Even my much, much larger PhotoShop apps load in 2-3 seconds.
You might have noticed that in said photo, my phone was in my hand. You can tell by the Stacks Bowers case it's in. An officemate took that shot and emailed it to me. By the way, Preview has always loaded like a pig since the day I got the Mac, brand new, from the Apple Store. So does iTunes, but I always chalked that up to having my library on a NAS.