Isn’t it sad that a man is behaving badly and your Dd is the one who doesn’t want to offend him.
I’d tell her that she should just go and sit near the driver as soon as creepy man sits down next to her. Maybe practice with her things she could say to him or driver. To driver, this man is making me uncomfortable. To creepy, stop following me or just Go Away. Act as if she is getting off bus earlier than the time before, when she is getting off bus, stand back, say After You to creep, wait and when he gets off bus, she stays on bus. She turns to driver and says He followed me off the bus last time.
When I was 10, I had a man come and sit next to me in a nearly empty cinema and put his hand on my knee. I was alone with my younger brother. I was too shy to do anything apart from making my brother leave halfway through the film.
when I was 17 I was travelling on a long coach trip and again a man came and sat next to me although there were plenty of spaces. He kept getting me to talk to him, but I was uncomfortable. When we took a rest stop I refused to going into the services with him and stayed on the bus.
The driver came to where I was sitting and asked if everything was ok. I explained the situation. He offered to tell the man to sit else where. Like your Dd I didn’t think I had the authority to tell the creepy man to get lost. But because the driver had noticed it gave me the confidence to tell mr creepy that I was going to have a sleep and needed both the seats. Because I knew the driver was watching, I felt much better.
So practice with your Dd how to say things, actually get her to say things in a loud clear voice, not the little mouse voice I would have used.
My experiences were long ago (and there were more). Nowadays, people are much more switched on about safeguarding. Try and help your Dd find her ability to call out bad behaviour. The sooner she finds it, the more safe she will stay.
Oh and she should get a picture of him, that may make him piss off.