Pedicabs will hold two people, that's pretty heavy duty.
I'm thinking take any basic "platform" type trailer, and fit a chair on it and there you go.
The setup you're describing doesn't really sound safe. A person, especially a full sized adult, sitting on a chair has a center of gravity that's much higher than is likely to safe on a standard trailer, even if you can find one that can support the weight.
I would think that something heavy duty would be exactly what you need. If you've ruled out rickshaws, how about a tandem?
Raleigh Aspen touring/off-road hybrid, and a Bob Yak trailer. Yak very useful for us car-free types that like to buy lots of beer.
That wicycle trailer is like what I had in mind. But notice it is only rated up to 150 lbs. I wonder what it would take to make it suitable for my 170 lb father. I don't want a rickshaw or pedicab. I already have a bike. I just want a trailer I can use to drive around an adult. Any more ideas?
I'm thinking there must be a way to convert a wheelchair into an adult trailer. Perhaps you can find one that has damaged or missing front wheels. You could attach some sort of connecting frame to the front of it.
(edit) If you could find an old athletic wheelchair, the kind with the inward leaning wheels and low center of gravity, that would be REALLY nice.
The bike doing the pulling better have an extra heavy duty frame and some bombproof wheels. You're asking a bike to handle your weight and an extra 170 lbs plus trailer weight. I doubt trailer weight ratings take into account what the bike can stand, just what the trailer can carry.
Not saying it can't be done, just that it would require the right bike.
Can your father ride a tandem?
Why can I not find anything about trailers to carry an adult passenger. Something similar to child trailers, but for adults instead. Any ideas?
I don't believe there is such a thing. The pedi cabs are probably better for some reason. I have seen pictures of wheelchair bikes with the wheelchair as the front of a bicycle. It may be that a bicycle plus an adult trailer is so heavy that you are better off with an inexpensive pedicab. It's probably less money to buy an inexpensive pedi cab than have a trailer made for you. It may ride better too. It will handle easier in traffic.
If there is no demand for a lot of them no one will produce them. Most people have no reason to carry an adult in a trailer.
There are many other rigs that will do the same thing. Side by side three wheelers, side by side four wheelers. And lots more. Tandem trikes.
There's a site on the web with a sort of basic trailer with a platform with 2 bike wheels but you can build them too.
Oh, here's an idea, see if you can score a decent wheelchair, like a Quickie. Then, attach a bar to it that attaches to your seat post. Make the wheelchair tilt back, it's up to you and your dad whether he wants to ride looking forward or looking back. That makes it a lot more stable, since by sitting back you'll get a "bucket seat" effect tending to keep the passenger in.
Most wheelchair users especially nowadays weigh more than 150 lbs so modern chairs are built sturdy enough, and I haven't priced them but they're probably not too much.
There's a maker in SF that builds pedicabs, and they have built a trailer version of the pedicab.
Bikes can handle that kind of weight--trailer weight is borne by the trailer, "tongue weight" is borne by the bike. My Bikes at Work trailer has a 300lb capacity. You want a good strong rear wheel though, like a 1" wide 26" suitable for heavy-duty riding. I use a Sun RhynoLite designed for MTB XC use.