Training Wheels for an Adult I have a friend that refuses to attempt to learn to ride a bike without training wheels/being a tricycle, despite being 22 years old and an avid jogger.....

I have an 30 year old touring bike, and a hybrid that is more on the mountain bike end of things.

I've noticed that adult training wheels are horribly expensive, and I don't have $200 to blow on a set.

I've come up with some crazy ideas that I might discuss with a professor from my engineering mechanics class, but does anyone have an affordable way to set up training wheels on a "typical", if you will, adult bike?
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.