What I always tell people who are new to the hobby is to pretend for a moment that you're a character from fiction and then recreate that person with your own spin. Even if all you do is figuratively file off the serial numbers and apply a coat of paint, you've still got a workable character. Uniqueness comes later; first, learn the basics.

Direct her to this page on TVTropes and tell her to have a look at the examples below the description. Between the descriptive text and the examples, she should have a good idea of what paladins are all about and what some of them in fiction are.

She can then pick and choose traits to build a plausible person from the base template.

After she's had some time to digest a little bit of paladins 'round the world, try a little bit of directed interview.

"So you want to be a Paladin eh, GF? Cool! So paladins are really good people in their own way. But all of them have something that's really important to them, like a god or a virtue or an idea. What do you think would be an interesting thing to have as your paladin's big important thing?"

Take what she says and work with her to expand on it. Suggest stuff that might work well in your game.

Personally I like characters that have some internal motivations: stuff they want in the short (weeks/months) medium (1-5 years) and long (5-20 years) term.

Also characters that have a good reason to be with the party (prevents or inhibits fracturing of the party).

If you can nail down those three components (paladin of what, motivations, and connection to the party) she should be able to figure out what to do on her own with a little guidance and an accepting attitude.