Training Wheels for an Adult
Don't.

Training wheels can be effective if someone has balance issues, but for those people a tricycle is normally a better approach.

To someone with reasonable balance learning to ride, using training wheels impedes learning. For infants, the balance bike toys are great, but they are not available in adult sizes. Instead, remove the cranks, bottom bracket, derailleur and chain from a normal bicycle. If you don't have the tools a bike shop should be able to do this cheaply. Ideally choose one that's a little on the small size for the adult so that the crossbar is lower and the handlebars closer than usual. Using a cheap/reclaimed bike means reassembling it afterwards isn't an issue, which is handy since old, cheap bikes are often really painful to put back together (and you'll probably end up buying new cables, chain and possibly bottom bracket anyway).

That setup lets them get the hang of balancing and steering with the safety of being able to put both feet on the ground easily. Do it off the street, obviously, preferably in a park with grass available to deal with the inevitable "what if I fall" question. Answer: try falling on the grass.