She can learn. But don't use training wheels. They're the worst. They don't teach riding at all. If anything, they teach the new rider that leaning causes problems, which is the exact opposite of what learning to ride entails.
Have her ride a "skut" type bike. That is, a bicycle without pedals, cranks and chain, and with a saddle low enough where the rider can plant both feet firmly on the ground at the same time. Self-balancing and pushing quickly teaches the new rider the dynamics of bicycle riding.
If a tandem isn't a good introduction to the feeling of riding a single, then put her on the back of a motorcycle. This may more accurately teach the untrained rider what it feels like to carve turns on two wheels.
She can learn. I taught myself how to ride a unicycle in 12 days (at age 50). But the trick to learning was devoting time to it every day. Your brain needs regular exposure to learn a new skill. Instead of, say, spending three frustrating hours every month, spend a half hour to 45 minutes every day. This is the trick. Don't have any expectations at all. Just go out and have her do her best. Don't care at all what sort of progress occurs. Her brain will absorb and process the things she needs to learn along the way. And I'd be very surprised if she isn't riding in relatively short order.
Once she learns the dynamics of bicycle riding, THEN it's up to you to make sure she gets the KNOWLEDGE involved to safely navigate the environment and ride safely. Learning proper braking technique is a perfect example. A bicycle can stop you quickly and safely, or it can throw you over the handlebars. Learning how to prevent the latter can mean the difference between happy cycling and never riding ever again.
Good luck!