Is there any particular reason to no look at side cars?
I'm not a fan of motorcycle conversions to street trikes. They don't have much storage space, and often even less than a motorcycle with side bags and a trunk. Those two large rear car tires and wheels really sap power. They're so common now that they hardly get a second look.
Out of the two you mentioned, the Tri Glide is the only production model. Honda does not sell trikes, all of the Goldwing trikes are aftermarket kits.
Has he test rode a Can Am Spyder? I've rode a Spyder and Tri Glide back-to-back and the Spyder is hands down better. It has more power, handles better and cost less. They can also be bought with a semi-automatic type of transmission or manual, and they have reverse.
They use Rotax engines, and they have been on the market long enough that nice used ones can be bought at reasonable prices.
If he's really wanting to stick with HD, his choices are limited to a trike kit or side car. Beyond that, he'll have to look at buying something else. Maybe he'd be a good future Ural owner. They've got a classic look and aren't made for blazing down the interstate, but enjoying the more scenic routes at lower speeds. Urals have been around for a long time and I occasionally see used ones for sale. Selectable two wheel drive Ural side car models are available too.
This is a 2015 Ural Gear-Up. Fuel injected, front rear disc brakes, leading link fork and selectable 2wd. They have several other models.
http://imz-ural.com/