How hard is it to train a dog to use a human toilet?

For safety reasons: It's impossible for a dog to balance on a toilet seat designed for humans. Imagine climbing on a toilet seat and squatting down to do your buisnes perched on top of the seat... now imagine having tiny and slippery dog paws. It's not a valid option.

Some friends had an elderly dog who started peeing in the shower when she became incontinent with old age. That was easy to clean, but might not be acceptable for everyone. The dog also started doing it on her own and wasn't trained to do so.

Consider what would cause her to pee inside. Ignore the specific place where she pees. She knows that she's supposed to pee outside, so what could cause her to do it in an inappropriate place?

• Dogs don't have the mental capacity to "want to be like" anyone. If she didn't observe anyone actually using the toilet, she probably doesn't even know what it's for. If there's no strong smell of urine in the room, she has no reason to assume that anyone ever pees there. Please forget your theory.
• If your dog does it again, get her checked for any problems or infections of the urinary tract. Those can cause pain when she needs to hold her pee.
• Did you ignore her requests to go outside for too long? If she's ignored too often, she might try different tactics because she assumes the usual one doesn't work anymore.
• Does she like to go outside in general? If being outside is always connected with negative experiences, like being scolded, she might want to avoid going outside at all.
• Have you been very stern with her? Urinating is a nonverbal communication indicating submission. Maybe she's very sensitive and has a delicate personality? If she starts peeing while or right after being reprimanded, you are too harsh with her.
• Was it just a one-time accident? No-one knows what's really going on in the head of a dog. Hopefully she just had a bad day and won't repeat this behavior.